This is similar/identical to this question - Signup with a custom code possible? - #2 by nicolas_sabena
Basically we’d like to control who signs up to our app by providing them with a code i.e. 12345 which they can use on sign-up. We are currently using the Universal Login page - it doesn’t look like we can add a custom field to this but that may have changed since the question above was asked.
Is there a way to implement this via auth0 or do we need to control this via our app, i.e. with a page users can enter the code before auth0 sign-up?