What is the best way to add a "donate" paypal button?

What I would do ...

I would get the URL method from PayPal.  Create your own button or take one from their site.

When you click the button in your app .. have stageWebView open within your app.. have it load the target page within your app.   This is a far better experience than leaving your app to go to Safari.

If you have a paypal account you can auto generate a donate button. Paypal will spit out the html code for you, in there is probably the link you want. Create a new button in your app and have it go to that URL? It would be nice to integrate it with the paypal app to make it easier for users.