Comma Logic for one of many fields with same name.

Hello there community!

I have been looking for a script that I can put into a text field that places a comma at the end of the text/ string. I found this little bit on another post: if (event.value) event.value+=","; this works 100% but I need to customize it a bit. I have 5 text fields that are identical in name that pull in information from a database but I only need that snippet of logic applied to the first instance of textbox. So, all 5 boxes will populate a client name but I only need comma logic applied to the first box. As it stands now, using just this tidbit in the first textbox produces a comma across all boxes which makes sense.   Any help or advice would be appreciated!