code: def subscribe_registration_ids_to_topic(self, registration_ids, topic_name):
          
          url = 'https://iid.googleapis.com/iid/v1:batchAdd'
          payload = {
              'to': '/topics/' + topic_name,
              'registration_tokens': registration_ids,
          }
          response = self.requests_session.post(url, json=payload)
          if response.status_code == 200:
              return True
          elif response.status_code == 400:
              error = response.json()
              raise InvalidDataError(error['error'])
          else:
              raise FCMError()
docs: Subscribes a list of registration ids to a topic
  
          Args:
              registration_ids (list): ids to be subscribed
              topic_name (str): name of topic
  
          Returns:
              True: if operation succeeded
  
          Raises:
              InvalidDataError: data sent to server was incorrectly formatted
              FCMError: an error occured on the server