FileMaker Pro 19 script issue

[Mrpower]

Hello all

I am not sure how many of you know FileMaker Pro scripting. I am mainly a beginner and know enough to get myself into a jam of misunderstanding. I will try to explain it as clearly as I can. I have two fields, first name & surname and I am putting them together to create a field called "full name". However, when the first name is edited, the full name field removes the surname. which means you have to enter the surname manually in to the field. I have scripted it to add from fields "first name & Surname" which works, ok ish, until edited, then above problem.

Anyone know how to solve this issue? Or anyone have any pointers? Or a wall I can bang my head against.

I did not create the file in the first place, I am building on someone else's foundations.