code: def previous_track(self):
          
          # Use previous track button only for sources which support NETAUDIO
          if self._input_func in self._netaudio_func_list:
              body = {"cmd0": "PutNetAudioCommand/CurUp",
                      "cmd1": "aspMainZone_WebUpdateStatus/",
                      "ZoneName": "MAIN ZONE"}
              try:
                  return bool(self.send_post_command(
                      self._urls.command_netaudio_post, body))
              except requests.exceptions.RequestException:
                  _LOGGER.error(
                      "Connection error: previous track command not sent.")
                  return False
docs: Send previous track command to receiver command via HTTP post.