How can I use the Add Product form as content in the main menu?
Admin > Structure > Menus > Main Menu > Add link


Title: Add product
Path: admin/commerce/products/add/product


Keep in mind, that is for admins to create backend products for the inventory, not front end product displays.

It will also only be generated for admins with the relevant permissions.

To add a link for creating a frontend product display, the path would be something like


node/add/product-display