Welcome to the Auth0 Community!
With a custom domain configured, you can make requests to your tenant by using either your canonical or custom domain.
The following domains are permitted:
https://YOUR_DOMAIN.REGION.auth0.com/
https://{{YOUR_CUSTOM_DOMAIN}}
However, when requesting a Management API access token, you must use the API identifier defined in your API settings, which has the following syntax:
https://YOUR_DOMAIN.REGION.auth0.com/api/v2/
I hope the explanation was clear!
Thanks,
Rueben