code: def add_user(self, user_id, custom_properties=None, headers=None, endpoint_url=None):
          
          endpoint_url = endpoint_url or self._endpoint_url
  
          url = endpoint_url + '/users'
  
          headers = headers or self._default_headers()
          payload = {"user_id": user_id}
  
          if custom_properties is not None:
              payload["user_properties"] = custom_properties
  
          response = requests.post(url, headers=headers, json=payload)
  
          return response
docs: Creates a new identified user if he doesn't exist.
  
          :param str user_id: identified user's ID
          :param dict custom_properties: user properties
          :param dict headers: custom request headers (if isn't set default values are used)
          :param str endpoint_url: where to send the request (if isn't set default value is used)
          :return: Response