In ASP.NET WebForms there is a limitation that a page can only have one html form and that form already exists, so it is not possible to add a paypal button that uses a form.

What you can do is create a paypal link the same as you would use for email and then wrap an image inside the link, this is how the Donate button on this site works.

Or you could possibly use the WebStore feature.

Hope it helps,

Joe