Welcome to the Auth0 Community, @mcodes !
As you’ve already mentioned, the feature that looks like would fit best to your use case would be a Post-User Registration hook. Have you tried troubleshooting the connection for example via our Real-time Webtask Logs Extension extension? What errors or problems are you facing?
Depending on your use case, you could potentially leverage a Create Script Action Script, but this is an Enterprise plan feature only as documented here: Pricing - Auth0