Cannot GET record for logged in USER

I am trying to filter a GET record request by appVars.currentUser.username. I want the logged-in user to be able to update his/her profile. The username is unique and in each the one record for the user, so I should be able to identify it, view it, and edit it. Struggling big time with filterByFormula and cannot crack this. Any help would be greatly appreciated.

Could you show here what your formula and GET record look like? And is it so that your users don’t have an id, but only username is unique?

!

I can create an ID for users, but had a hard time linking to appVars.currentUser.id through the AG auth system.