How do I create a trigger which fires when a requester adds a comment?

I need to create a trigger which adds a tag to a ticket when another Zendesk connected support email address is CC'd.

I tried using the "Ticket Is Updated" trigger condition, but that did not work. I believe that "Updated" only triggers when an agent submits a ticket update, not when a ticket is updated by an outside requester comment.

What trigger condition should I use to fire when a requester updates a ticket by email?