@Victor I am doing it from the other side with my existing FileMaker Database. Posting users from my App to the Claris FileMaker. But once they have their UUIDs and the Element ID will be available (@David & @anon78309838 
) it’d be easy.
For Victoires Zap I cannot see from what we have here, why it would not work. I mean, you can find the user and matching the fields is trivial. What are we missing?