I don’t understand how shopify is so inept with this basic feature - I had to look through 20 different articles to explain this in simple terms.
To find the variant id of a product with no variants: in shopify backend goto Products > Click the product you want to add > Now add .xml to the end of the url so store.com/admin/products/id.xml
From the xml page, look for ######### - This is the number you need to add instead of the product number.
Thanks for not clarifying this anywhere, shopify.