code: func (api *Client) AddPinContext(ctx context.Context, channel string, item ItemRef) error {
  	values := url.Values{
  		"channel": {channel},
  		"token":   {api.token},
  	}
  	if item.Timestamp != "" {
  		values.Set("timestamp", item.Timestamp)
  	}
  	if item.File != "" {
  		values.Set("file", item.File)
  	}
  	if item.Comment != "" {
  		values.Set("file_comment", item.Comment)
  	}
  
  	response := &SlackResponse{}
  	if err := api.postMethod(ctx, "pins.add", values, response); err != nil {
  		return err
  	}
  
  	return response.Err()
  }
docs: // AddPinContext pins an item in a channel with a custom context