NO IDEA WHAT I AM DOING- Subforms

NO IDEA WHAT I AM DOING- Subforms

I want a field in my subform (Cust_Ref) to automatically fill from the given input in the main form (Claim_Number)

I have tried using a number of things in the script builder but I can't get ANYTHING to work.

I need to know how to write the code and WHERE EXACTLY to put it. I am at my wits end. Nothing I have entered has even been saved by the script builder (I am hitting save script and getting a success message)... when I go to check to see if it worked and go back to the builder the code I entered has disappeared.