Adding a new Product involves creating both a Product Item and a SKU Item, since a Product Item has to have, at minimum, a SKU Item.
To create a new Product with multiple SKUs, you must:
Create the Product and Default SKU using this endpoint, making sure to add sku-properties in the product data.
You can't add sku-values to the SKU yet, since there are no enum IDs created yet. When this endpoint returns, it will have IDs filled in for the sku-properties enums.
With those IDs, update the default SKU with valid sku-values and create any additional SKUs (if needed), with valid sku-values.
You can also create the Product without sku-properties and add them in later.
If you add any sku properties, the default SKU will default to the first value of each option.
Upon creation, the default product type will be Advanced. The product type is used to determine which Product and SKU fields are shown to users in the Designer and the Editor. Setting it to Advanced ensures that all Product and SKU fields will be shown. The product type can be edited in the Designer or the Editor.