Extra validation username field in sign up form Login by Auth0 wordpress plugin

I mean Auth0 Hooks, not wordpress hooks…

Where can I find that js file? I’m using wordpress plugin. Can you give me more details, please?

As I can see here:

Rules execute after the initial authentication step is completed; for a database connection user that would mean that the user provided correct credentials (username/email and password); they do not execute at user signup.
If your use case is around database connections you should check the pre-registration hook