Prompt user for file name - scanning to server

I have a WC 7556.

I have configured the workflow scanning to scan, and save the result  to a file on our server. 

The problem I need to solve is how to prompt for a custom name for that file.  The file name needs to be say, a custom file identification code, say M79866.

I can see where the custom fields are dates, job time, etc., but the user needs to be prompted for the above identification code.

Is there anyway to do that?

Thanks,

Captainkey