code: func (api *Client) MarkIMChannelContext(ctx context.Context, channel, ts string) error {
  	values := url.Values{
  		"token":   {api.token},
  		"channel": {channel},
  		"ts":      {ts},
  	}
  
  	_, err := api.imRequest(ctx, "im.mark", values)
  	return err
  }
docs: // MarkIMChannelContext sets the read mark of a direct message channel to a specific point with a custom context