Add a custom field to the users metadata with the Pre User Registration trigger.
In the Actions editor enter the following code
exports.onExecutePreUserRegistration = async (event, api) => {
api.user.setUserMetadata("favorite_ninja_turtle","raphael" )
};
This action uses the API object to add the new metadata before the user is saved to the database
To learn more about Auth0 user metadata read the docs here