What is the best way to handle external event registration within a web application?

My organization's platform has two types of events that can be created by admin users: internal and external. If the event is internal, the registration is completed within our web application and we can keep track of how many end users RSVP.

My question lies with events where registration is done on an external website, like Eventbrite. Our admin users don't manage these events, but use our platform to promote them to their end users. However, we would also like to keep track of the end users who have signed up in order to send any notifications regarding updates to the event.

What is the best UX to keep track of users who signed up on the external site?