Script to add value to a ticket's multi-select field if it is not already there

Script to add value to a ticket's multi-select field if it is not already there

Here's the scenario:

Ticket layout has a multi-select field cf_resources_used with these options available as a start: L3, Cloud, Specialist.

Its purpose is to help us track the resource(s) we consult with on a ticket.  To consult with a resource, the agent will apply that resource's macro which will change the ticket status, add the resource to cf_resources_used, and do other things.

Example: If the agent needs L3, they apply the L3 macro which will change status to With L3 and add L3 to cf_resources_used if it isn't already there. 

Assume my function already has resource_name = "L3". How do I:
  1. test if the ticket's cf_resources_used field contains L3 already
  2. add L3 to the field if it is not there

Also, did I select the right subcategory for this question? Not sure if it should be in developer APIs or Using Zoho Desk.

Thanks!
Mel