Question:
Currently setting a custom field as "Mandatory" means - it must be populated when the ticket is first created and when a ticket is closed. We need to have a custom field which is only required to be filled in before the ticket can be closed.
Answer:
Add a mandatory custom field and also make it viewable to technicians only. This way regular users won't see this field when creating a ticket, but technicians will be prompted to set it, before they're able to close a ticket.