Hello Vicki,
The Contacts table is the one table that is automatically linked to the List Builder. Check out this forum post for more info. It can be feasible using the Create Short Method URL action on the Onscreen Load action. Take a look at the screen shot below. The idea is to create a field on the List Builder table to hold the short URL once it is created. Then it should be available for the email templates.
For the action, you can start with the plain URL to the login screen. The trick is seeing if the optional fields can be obtained. They allow the user to go directly to the transaction once logged into the portal.
Greg