If you want a "donate button" in your application, you can go to paypal and go through the "create a donate" button setup. In the end you choose to create an email link. This means that paypal will store most things on their side regarding your button. Then you create a button in your app (e.g. in the about box) with the paypal/donate icon and connect it to the email link paypal just gave you for use in emails. IMHO that's the easiest way to link from you app to a paypal donation.

The result: Your app has a donate button and once a user clicks it the browser will open with everything in place to directly donate. Osm2Go and Gpxview both have this tiny button in their about boxes. Feel free to "steal" that feature and/or improve it.