Fill in field based on query

Hi, I have a question about how to auto-fill a field.

My database has a User table, with login form. When a user logs in it creates a tempvar which sets the username. This username is shown on every form through a query with dlookup. This works fine.

So now i want to know which user added a deal.

In my "Deals" table i've added a field called "Username". I want to autofill this field based on this tempvar or query and this information must be saved in the deal table. I've copied the textbox which shows the username and changed the control source to the right table. But when I link it to the table it stops showing the username.

So, my question: how can I make it that the textbox grabs the username from the tempvar or query and when I press my save button that this information is stored in the table?

Thanks in advance :)