Willy,
You have to edit the CRM_ListBuilder screen (make a copy and edit the copy). You're looking for the process buttons toward the bottom. There's an action on those buttons that creates the URL in the email. There's a number of actions, so you'll have to search for the one that creates the URL. If you edit that action, you'll see the screen shot below. The highlighted section is where you can change the screen that opens in the portal.
- Adam