code: def add_payload(self, key, val, append=False):
          
          self._request.add_payload(key, val, append)
docs: Add a key value pair to payload for this request.
  
          .. Note:: For ``_search`` you can pass a search argument. (e.g. _search?summary=1.1.1.1).
  
          Args:
              key (string): The payload key
              val (string): The payload value
              append (bool): Indicate whether the value should be appended