How to save user_metadata in Custom Database?

How can i save additional fields to a custom mysql database and update them using the management api?

The custom database is meant to provide you with the ability to maintain the user credentials in your own existing store; in addition, you can then decide to surface some additional user profile information from the existing store into the Auth0 user profile. However, it is not meant to a provide a two-way communication channel for dynamic information like user metadata so there is no support to reflect changes to user metadata triggered from the Management API back to the underlying user store.