Hey @aj.feather,
Thanks for clarifying your use case—and apologies, you’re correct in that whether or not registration is required will affect the behavior here.
I’ve followed up with our team on this scenario and have raised the importance of being able to more easily map IDs from the List Registrants endpoint to the participant joined webhook. (ZOOM-181169). While your best bet for now will be to work off of the email field, I understand this doesn’t capture every case, and will be happy to advocate for an improved experience here.
Thanks again for raising this, AJ!
-Will