Hi Jason,
Once a contact record is created from Event page, its Originating event field will be populated with the event.
(This section is in Details tab)

Also, a Event Registration record will be created, which saves contact and contact's registered Event.

In my example, I registered 2 contacts from event Ted Contoso.
So we could build your required segment in 2 ways:
1. Build a segment and set Contact as source entity, set contact's originating event field to the specific event we want:

2. Build a segment and set Event Registration as source entity, set its Event field to the specific event we want, then find its related contacts.

Finally add any of segment into customer journey for test:

Result:

Regards,
Clofly