Hey Caitlin. To be precise, it isn't a trigger, it's a filter. It has to be a filter to work in an Engagement Program (if I remember training from 14 months ago correctly).
You're right to question the last bit. I wasn't very descriptive. A screenshot is much easier.
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Be careful with this though & check your logic that you don't require someone to be a member of the program. If i wanted to use this filter for this engagement program example, I'd also add in a filter & the filter logic (ANY) at top. I'd also use Member of Program (False) to ensure that you can both NOT be a member or be a member without the success.