source: contact-form-7/tags/5.9.6/modules/sendinblue/contact-form-properties.php

<?php

add_filter(
        'wpcf7_pre_construct_contact_form_properties',
        'wpcf7_sendinblue_register_property',
        10, 2
);

/**
 * Registers the sendinblue contact form property.
 */
function wpcf7_sendinblue_register_property( $properties, $contact_form ) {
        $service = WPCF7_Sendinblue::get_instance();

        if ( $service->is_active() ) {
                $properties += array(
                        'sendinblue' => array(),
                );
        }

        return $properties;
}


add_action(
        'wpcf7_save_contact_form',
        'wpcf7_sendinblue_save_contact_form',
        10, 3
);

/**
 * Saves the sendinblue property value.
 */
function wpcf7_sendinblue_save_contact_form( $contact_form, $args, $context ) {
        $service = WPCF7_Sendinblue::get_instance();

        if ( ! $service->is_active() ) {
                return;
        }

        $prop = (array) ( $_POST['wpcf7-sendinblue'] ?? array() );

        $prop = wp_parse_args(
                $prop,
                array(
                        'enable_contact_list' => false,
                        'contact_lists' => array(),
                        'enable_transactional_email' => false,
                        'email_template' => 0,
                )
        );

        $prop['contact_lists'] = array_map( 'absint', $prop['contact_lists'] );

        $prop['email_template'] = absint( $prop['email_template'] );

        $contact_form->set_properties( array(
                'sendinblue' => $prop,
        ) );
}


add_filter(
        'wpcf7_editor_panels',
        'wpcf7_sendinblue_editor_panels',
        10, 1
);

/**
 * Builds the editor panel for the sendinblue property.
 */
function wpcf7_sendinblue_editor_panels( $panels ) {
        $service = WPCF7_Sendinblue::get_instance();

        if ( ! $service->is_active() ) {
                return $panels;
        }

        $contact_form = WPCF7_ContactForm::get_current();

        $prop = wp_parse_args(
                $contact_form->prop( 'sendinblue' ),
                array(
                        'enable_contact_list' => false,
                        'contact_lists' => array(),
                        'enable_transactional_email' => false,
                        'email_template' => 0,
                )
        );

        $editor_panel = static function () use ( $prop, $service ) {

                $description = sprintf(
                        esc_html(
                                __( "You can set up the Brevo integration here. For details, see %s.", 'contact-form-7' )
                        ),
                        wpcf7_link(
                                __( 'https://contactform7.com/sendinblue-integration/', 'contact-form-7' ),
                                __( 'Brevo integration', 'contact-form-7' )
                        )
                );

                $lists = wpcf7_sendinblue_get_lists();
                $templates = $service->get_templates();

?>
<h2><?php echo esc_html( __( 'Brevo', 'contact-form-7' ) ); ?></h2>

<fieldset>
        <legend><?php echo $description; ?></legend>

        <table class="form-table" role="presentation">
                <tbody>
                        <tr class="<?php echo $prop['enable_contact_list'] ? '' : 'inactive'; ?>">
                                <th scope="row">
                <?php

                echo esc_html( __( 'Contact lists', 'contact-form-7' ) );

                ?>
                                </th>
                                <td>
                                        <fieldset>
                                                <legend class="screen-reader-text">
                <?php

                echo esc_html( __( 'Contact lists', 'contact-form-7' ) );

                ?>
                                                </legend>
                                                <label for="wpcf7-sendinblue-enable-contact-list">
                                                        <input type="checkbox" name="wpcf7-sendinblue[enable_contact_list]" id="wpcf7-sendinblue-enable-contact-list" value="1" <?php checked( $prop['enable_contact_list'] ); ?> />
                <?php

                echo esc_html(
                        __( "Add form submitters to your contact lists", 'contact-form-7' )
                );

                ?>
                                                </label>
                                        </fieldset>
                                </td>
                        </tr>
                        <tr>
                                <th scope="row"></th>
                                <td>
                                        <fieldset>
                <?php

                if ( $lists ) {
                        echo sprintf(
                                '<legend>%1$s</legend>',
                                esc_html( __( 'Select lists to which contacts are added:', 'contact-form-7' ) )
                        );

                        echo '<ul>';

                        foreach ( $lists as $list ) {
                                echo sprintf(
                                        '<li><label><input %1$s /> %2$s</label></li>',
                                        wpcf7_format_atts( array(
                                                'type' => 'checkbox',
                                                'name' => 'wpcf7-sendinblue[contact_lists][]',
                                                'value' => $list['id'],
                                                'checked' => in_array( $list['id'], $prop['contact_lists'] ),
                                        ) ),
                                        esc_html( $list['name'] )
                                );
                        }

                        echo '</ul>';
                } else {
                        echo sprintf(
                                '<legend>%1$s</legend>',
                                esc_html( __( 'You have no contact list yet.', 'contact-form-7' ) )
                        );
                }

                ?>
                                        </fieldset>
                <?php

                echo sprintf(
                        '<p><a %1$s>%2$s <span class="screen-reader-text">%3$s</span><span aria-hidden="true" class="dashicons dashicons-external"></span></a></p>',
                        wpcf7_format_atts( array(
                                'href' => 'https://my.sendinblue.com/lists',
                                'target' => '_blank',
                                'rel' => 'external noreferrer noopener',
                        ) ),
                        esc_html( __( 'Manage your contact lists', 'contact-form-7' ) ),
                        esc_html( __( '(opens in a new tab)', 'contact-form-7' ) )
                );

                ?>
                                </td>
                        </tr>
                        <tr class="<?php echo $prop['enable_transactional_email'] ? '' : 'inactive'; ?>">
                                <th scope="row">
                <?php

                echo esc_html( __( 'Welcome email', 'contact-form-7' ) );

                ?>
                                </th>
                                <td>
                                        <fieldset>
                                                <legend class="screen-reader-text">
                <?php

                echo esc_html( __( 'Welcome email', 'contact-form-7' ) );

                ?>
                                                </legend>
                                                <label for="wpcf7-sendinblue-enable-transactional-email">
                                                        <input type="checkbox" name="wpcf7-sendinblue[enable_transactional_email]" id="wpcf7-sendinblue-enable-transactional-email" value="1" <?php checked( $prop['enable_transactional_email'] ); ?> />
                <?php

                echo esc_html(
                        __( "Send a welcome email to new contacts", 'contact-form-7' )
                );

                ?>
                                                </label>
                                        </fieldset>
                                </td>
                        </tr>
                        <tr>
                                <th scope="row"></th>
                                <td>
                                        <fieldset>
                <?php

                if ( $templates ) {
                        echo sprintf(
                                '<legend>%1$s</legend>',
                                esc_html( __( 'Select an email template:', 'contact-form-7' ) )
                        );

                        echo '<select name="wpcf7-sendinblue[email_template]">';

                        echo sprintf(
                                '<option %1$s>%2$s</option>',
                                wpcf7_format_atts( array(
                                        'value' => 0,
                                        'selected' => 0 === $prop['email_template'],
                                ) ),
                                esc_html( __( '&mdash; Select &mdash;', 'contact-form-7' ) )
                        );

                        foreach ( $templates as $template ) {
                                echo sprintf(
                                        '<option %1$s>%2$s</option>',
                                        wpcf7_format_atts( array(
                                                'value' => $template['id'],
                                                'selected' => $prop['email_template'] === $template['id'],
                                        ) ),
                                        esc_html( $template['name'] )
                                );
                        }

                        echo '</select>';
                } else {
                        echo sprintf(
                                '<legend>%1$s</legend>',
                                esc_html( __( 'You have no active email template yet.', 'contact-form-7' ) )
                        );
                }

                ?>
                                        </fieldset>
                <?php

                echo sprintf(
                        '<p><a %1$s>%2$s <span class="screen-reader-text">%3$s</span><span aria-hidden="true" class="dashicons dashicons-external"></span></a></p>',
                        wpcf7_format_atts( array(
                                'href' => 'https://my.sendinblue.com/camp/lists/template',
                                'target' => '_blank',
                                'rel' => 'external noreferrer noopener',
                        ) ),
                        esc_html( __( 'Manage your email templates', 'contact-form-7' ) ),
                        esc_html( __( '(opens in a new tab)', 'contact-form-7' ) )
                );

                ?>
                                </td>
                        </tr>
                </tbody>
        </table>
</fieldset>
<?php
        };

        $panels += array(
                'sendinblue-panel' => array(
                        'title' => __( 'Brevo', 'contact-form-7' ),
                        'callback' => $editor_panel,
                ),
        );

        return $panels;
}


/**
 * Retrieves contact lists from Brevo's database.
 */
function wpcf7_sendinblue_get_lists() {
        static $lists = array();

        $service = WPCF7_Sendinblue::get_instance();

        if ( ! empty( $lists ) or ! $service->is_active() ) {
                return $lists;
        }

        $limit = 50;
        $offset = 0;

        while ( count( $lists ) < $limit * 10 ) {
                $lists_next = (array) $service->get_lists( array(
                        'limit' => $limit,
                        'offset' => $offset,
                ) );

                if ( ! empty( $lists_next ) ) {
                        $lists = array_merge( $lists, $lists_next );
                }

                if ( count( $lists_next ) < $limit ) {
                        break;
                }

                $offset += $limit;
        }

        return $lists;
}