Hi,
Subscribe in a trigger is not “add this customer as a watcher”. It only manages agent mentions / ticket subscriptions. Customers cannot be subscribed.
A mentioned user also needs agent access to the ticket’s group. If the selected user is a customer (or an agent without group access), the trigger does nothing useful.
So if the “particular customer” in the action is really a customer record, that setup cannot work. There is no stock trigger action that auto-subscribes a customer.
What does work:
- Subscribe a specific agent who has access to that group.
- Or notify that person with a trigger email if they only need to be informed.
If you actually selected an agent and it still does not stick: check the trigger activator (action = created vs always), that the organisation is already set at create time, and production.log for the mention/group-access validation.