Hey @pkreemer, thanks for posting and using Zoom!
Have you tried adding your app’s user ID as a query param to the join url and seeing if it persists and is included in the Webhook?
Otherwise I would set the users name as the user ID, or enable registration and add a custom question which your app can add the user ID too automatically (all possible with API, save response in db with registrant_id as primary key).
Then after the user joins you can use the user_id in your Participant Joined Webhook and match it with the registrant_id (returned from response of Add Meeting Registrant) to know the participants custom userID.
Let me know if that helps!
Thanks,
Tommy