I want to store the user reference once the user validates their email. So when a user clicks on the validation link in their email, I wanted to create a reference of that user to my external DB, like Auth0 ID or something.
You can send the user ID to your own API in a Post-User Registration Hook or a Rule.
If your app only has a database connection (i.e. email/password login), you can use a Post-User Registration Hook to send the user ID to an external API.
However, the Post-User Registration Hook will only fire for database connections. If your app has multiple types of connections such as social connections (“Sign in with Google”) and a database connection (email/password), then you can use a Rule to do the same thing.