Trigger when requestor (customer) updates a ticket with new response?

I thought this one would be super obvious, but I can't seem to find an appropriate trigger from the drop down.

Use case: whenever an existing ticket is updated with a new message from the requester (not from any of the agents) it triggers a Slack message into a channel to notify an agent that they should check the new response in Zendesk. 

What I've tried so far:

Assignee | is | [agent's name]
Ticket status | Changed to | Open
Updated via | is | Web form
Updated via | is | Email

But this does not work. Also, it doesn't matter if the customer updates the ticket via email or any other method, I just want the update itself to be the trigger.

Any ideas?