Creating IPN Auto Reply Using PayPal Donation

I have created a Donate button using PayPal because I want unregistered users (one-time visitors) to be able to specify the amount they want to contribute to gain access to a password protected page. In the auto-reply, I want a link to the page as well as the password to be sent in it. I have the button figured out, but I don’t know what’s the code that I should incorporate with it that would trigger the IPN response that I’ve set up in S2Member. I’ve searched around the forums, but the responses are not quite what I’m looking for. I need to figure out how to setup an IPN auto-email reply after someone makes a donation.