auth0 Custom Database Action Script - How to access user_id?

Question

I'm using auth0, I'm making a script on create under Action Scripts to save user to my mongoDB database. I only need to save the email and user_id, but I can't find the user_id?

Code is below, but the user only returns: {"tenant":"tenant-name","connection":"MongoDB-MainDB","email":"xxx","password":"xxx","debug":true}

Here is my code (This is basically the sample code provided by auth0):

function create (user, callback) {
  mongo('mongodb://xxx', function (db) {
    var users = db.collection('users');

    users.findOne({ email: user.email }, function (err, withSameMail) {

      if (err) return callback(err);
      if (withSameMail) return callback(new Error('the user already exists'));
      else {
        users.insert({ "_id" : user.user_id, "email" : user.email}, function (err) {
        if (err) return callback(err);
        callback(new Error(JSON.stringify(user)))
        });
      }
    });
  });
}

auth0 tutorial for reference: https://auth0.com/docs/connections/database/custom-db