Customer Portal User ID, Username and Email

Customer Portal User ID, Username and Email


Good day,

I have two forms each with a User field (with Customers), In the first form I setup an alert that shows me the input.Customer when I pick one in the dropdown list, it shows me the customer ID, with this ID I want to fetch a record in the second form where the same customer is selected. 

The problem is that in the storage record the User field takes a string value with the Username (or User Email). So I can't complete the fetch sucessfully because in my first form I have an ID.
 
How should I do this fetch?