post https://api.fabric.inc/v3/products//variants/actions/attach
Variants are the different options of an item such as size, color, and materials. For example, there are 12 variants for a laptop that comes in three sizes (13, 15, and 17 inches) and four colors (red, blue, grey, and white). By adding variants to products, you can offer more options to shoppers.
With this endpoint, you can add up to 25 variants to an existing product.
Note:
1. At least one variant must be specified.
2. If you don't have product ID, use the corresponding SKU-based endpoint - POST /products/sku/{sku}/variants/actions/attach.