Trigger for Autonumber script

Trigger for Autonumber script

I have a auto-number field that I want to populate only when a checkbox is selected.

USE CASE:

Customer files a complaint and want to return goods.  All complaints are logged, but not all goods are authorized for a return.  So I have a checkbox (called RMA Issued?) that indicates a Return is being authorized.

When that checkbox is selected, I want zoho to create a sequential number in a field (RMA-100000) for example.

If that checkbox is not selected, I don't want a number created....even if it's hidden....because I want the next valid number to be in sequence.

So I think I'm looking for a script to put in a workflow?