Name your Journey (A) ( We recommend you prefix with your department, in my case IT).
Select Trigger Based (B) and Select or Create your trigger (C) and add Condition(see below) (D) if desired. Hit Create (E).
The condition below is that the Event Registration is for an Event where the Event Name has the term “TEST” included.
The Trigger which placed them on this Journey (A)
The event registration entry which was created (B)
The parent event for which the event registration belongs (C)
The Event Name field on the Event (D) contains (E) “TEST” (F)
You can select any field, argument, value combination you need to ensure that you have the correct audience for your journey.