Add text input field for keyword(s) to PPD
 

I would like to edit a PPD to add one or more text input fields, to add keywords to the print job. The keywords would not affect the execution of the job in the printer, but would be useful for accounting purposes. I can see how to edit a PPD to add "pick one" lists, but not allow the entry of strings. How is this done? Are any examples available?

I can not help much except refer you to the cups documentation which has some examples that may or may not be helpful. Hopefully the application you are running will use the custom extensions but not sure exactly how to use the custom extension except to create a custom filter.

https://www.cups.org/doc/spec-ppd.html#OPTIONS