Form field value based on 2 other fields

Docs show how to generate the value of a slug field. And shoes how to test against the old value of the field before changing it.

How to fill the value of a field based on the values of 2 other fields?

And only fill the third field if it is empty: don't change it if it already has value.

Solution

Solved:
My problem was in the Helper function. It wasn't returning.