code: func (h *Client) PromotionID(promotionID string) *Client {
  	h.promotionID = promotionID
  	h.promotionIDSet = true
  	return h
  }
docs: // The promotion ID. Promotion index must be a positive integer
  // between 1 and 200, inclusive. For analytics.js the Enhanced
  // Ecommerce plugin must be installed before using this field.