New field that populates the user name from a field, but is changeable through a dropdown list.

There are a couple of choices.

One would be to have a form rule which says that when the Chosen Requestor is blank, change the Chose Requestor to the value in the field Requestor.  Set the rule to fire all the time and not just when the fields change, so uncheck that box at the bottom of the form rule.
https://help.quickbase.com/user-assistance/form_rules.html

The other option is to essentially do the same thing with an ACTION.
https://help.quickbase.com/user-assistance/creating_a_quickbase_action.html