docs: def add_registration(self, username, first_name, last_name, email, password=""):
          """
              Add a registration request for the user.
  
          :rtype : RegisterUser
          """
          register_user = self.appbuilder.sm.add_register_user(
              username, first_name, last_name, email, password
          )
          if register_user:
              if self.send_email(register_user):
                  flash(as_unicode(self.message), "info")
                  return register_user
              else:
                  flash(as_unicode(self.error_message), "danger")
                  self.appbuilder.sm.del_register_user(register_user)
                  return None