change field value button

change field value button

I have a simple application which I want to be used in production. 

I am stuck on creating a simple script that when push Vinde custom action button adds the letter " a" before Cod produs field value. From the view I set up a custom action which shows for each record. I just want people to click Vinde button and it will change the  Cod produs field value from 1001 to a1001.
Thanks anyone who can help!