Currently, when an agent adds a form to the request and marks it "External" for the reporter to complete or when the agent marks an existing form on the ticket as "External" for the report to add/edit information, there does not appear the be an event trigger associated with this action.

There is also no automation trigger related to this event. The only automation trigger related to forms is when they are submitted. 

It is important that the reporter gets notified that a form is being sent to them. This is especially important because anything related to forms does not show in the issue's history as well and does not trigger any issue change events.

Workaround:

The agent can add an external comment after they make the form External. This will trigger a customer notification for public comment added (if it is enabled). In this way, the end user gets notified that there is a form to complete.