Change affiliate generated link

I have my affiliate program all set up. The one last thing I can't figure out how to do is change the page that the affiliate links go to. For example, when a user registers to be an affiliate and it generates their unique link, it creates a link that looks like:

http://www.mydomain.com?ref=username-id

but i need it to go to

http://www.mydomain.com/thisproductpage?ref=username-id

I see how I can do that as long as the affiliate uses a banner, but I need it fixed to be in their generated URL, also. Thanks!