Update one User field when another one is updated

I have a app that shows who the current user is based on a State selected.

In order to make sure I record the correct user at the time the record is created. I have created another field let's say user at time of creation.  When a new record is created I want the user in the user field to update the user in the user at time of creation field.

I put a form rule that says if User at time of creation is null, update user at time of creation with user in user field.  However it is not working.

Any ideas?

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Carol Mcconnell
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