How do I reference a custom Desk field in a PUT statement

How do I reference a custom Desk field in a PUT statement

This works for any OOTB Desk field I try, such as "priority":

      resourceMap = map();
      resourceMap.put("priority","Low");
      zoho.desk.update(ORGID, "tickets",TicketID,resourceMap);


But it doesn't work if I use this or any other custom field:

      resourceMap.put("cf_resources_used","L3");

I can't find any doc or examples of this. Anyone have advice? 

Thanks,
Mel.