You would want to use something like this:
Code:string paypalurl = "https://www.paypal.com/cgi-bin/webscr?cmd=_donations&item_name=donate&business=youremail@domain.com&amount=1.00¤cy_code=USD";
Response.Redirect(paypalurl);
You just have to change the variables in the URL to apply to donations instead of a shopping cart.
You can check out the variables
here.