The partner company provides all support for promotional codes for the associated product. For example, if the code came with purchasing a controller, the company you bought the controller from would provide the support. Forza Support cannot offer replacement codes or resolve issues with redeeming them.
Promotional codes that unlock content in Forza games can be included with a wide variety of products like toys, peripherals, snacks, etc...
Support for a service code, like an Xbox Game Pass 14-day trial or Xbox Gold, is handled through Xbox Support.