That trigger is custom based on the form button created in the search form that was created.
The form uses GET instead of POST to allow for bookmarking of the search results and prevents resetting of the results when navigating or submitting a form not related to the search form.
That trigger is not listed, but if you were to go to the bindings panel and add the search form from the search page to the bindings then you could add it again by using the lightning bolt and choosing the button as the trigger from the submitted search form you added.