code: func (api *Client) StartRTMContext(ctx context.Context) (info *Info, websocketURL string, err error) {
  	response := &infoResponseFull{}
  	err = api.postMethod(ctx, "rtm.start", url.Values{"token": {api.token}}, response)
  	if err != nil {
  		return nil, "", err
  	}
  
  	api.Debugln("Using URL:", response.Info.URL)
  	return &response.Info, response.Info.URL, response.Err()
  }
docs: // StartRTMContext calls the "rtm.start" endpoint and returns the provided URL and the full Info block with a custom context.
  //
  // To have a fully managed Websocket connection, use `NewRTM`, and call `ManageConnection()` on it.