@stefan2013,
One way to accomplish this would be to use a WordPress page rather than writing your own. To add Subscribe2 to that page simply insert the shortcode.
If you are writing your own page outside WordPress for some reason then adding Subscribe2 to that would involving attempting to call many elements from within the WordPress API so would probably not be simple.
From the official plugin page (http://wordpress.org/extend/plugins/subscribe2/installation/):
Create a WordPress Page to display the subscription form. When creating the page, you may click the “S2” button on the QuickBar to automatically insert the Subscribe2 token. Or, if you prefer, you may manually insert the Subscribe2 shortcode or token: [subscribe2] or the HTML invisible <!–subscribe2–> Ensure the token is on a line by itself and that it has a blank line above and below. This token will automatically be replaced by dynamic subscription information and will display all forms and messages as necessary.
@mattyrob
Today I just had the same problem but I remember to check the plugin page. Anyway I think it’s useful if you could insert a small text in the miscellaneous admin page (or somewhere else) to remember people how they can implement the plugin (the widget, the shortcode and the html mode,…)
