you can use the Enviroment.SpecialFolder enum, although depending on your requirements you might lookslook at creating a windows service instead of an app that has to start on startup.
File.Copy("shortcut path...", Environment.GetFolderPath(Environment.SpecialFolder.Startup) + shorcutname);
edit:
File.Copy just takes aneeds an origin file fromdirectory-path and file to paththe target directory-path to copy a file. The key in that snippet is Enviroment.GetFolderPath(Enviroment.SpecialFolder.Startup) which is getting the startup folder path where you want to copy your file to.
you could use the above code several ways. If you hadIn case you've got an installer project for your app, you could run something like this on install. Another way could be when the app launches it checks if the shorcut exists there and puts one there if not (File.Exists()).
Here is a question about creating shortcuts in code also.