Can reference variables be created dynamically?

I have a user table which is what gets created when a user signs up on my site. A user in this case is a teacher. Then I have a readers table which gets updated for every reader the user adds from the classroom. Now I need to map a bunch of readers to the same user. So I created a reader_user_mapping table with just two fields readerId and UserId to map them. I reference the user Id to the User table and I am trying to dynamically populate this mapping table every time a row gets inserted into the reader table.
The problem is that the dynamic population of data works for fields other than the reference variable.
Question to the Wix gang is- Do Reference variables only work when they are pre-populated into the database. Is there a way to make them work when the data is added dynamically from the website forms.