How to create a metafield for a product variant on Shopify?

I want to create metafields for a product variant but not sure how to do that.
Actually, the product is a book so it doesn’t have a variant but Shopify creates a default title variant product for any product created on it. And I want to associate some information with that variant on Shopify.

I am looking for both technical ways and UI(if it exists). Need your help.