The problem is that it uses Stripe Connect so you don’t get your own Stripe api keys that the Discourse Subscription plugin still needs.
This webhook likely doesn’t match the Stripe one so it won’t be compatible with the Discourse subscription plugin.
The discourse subscription plugin likely isn’t the route to go for this at the moment because it is only designed to work with Stripe directly. Unless you want to build a simple Discourse plugin yourself to accept the thrive webhook I would look into using Zapier It should be able to accept the Thrive webhook, and then make a corresponding api call to Discourse that adds the user to your private group.