we have created a serverless app that has an on ticket update and external event elements to it.
The update of a custom field, triggers an automation which calls a webhook to trigger the external event. This then triggers the onticketupdate in the app which then adds a note to the ticket (we have used this to get the app working and will then do the required updates), it all works on the local machine using fdk run and the serverless ap works with adding a note to the ticket. the challenge we are facing is getting the webhook url for the external event to trigger correctly.
For custom fields or events based on custom fields, they do not trigger serverless events by default. They need to be explicitly enabled via the Dev Assist request using the link I shared before.
In this case based on what you mentioned, you need to trigger an event on custom field, which will result in action that triggers the webhook call. So first you may need to enable this setting for your account if not done already.