I bought a refurbished tower with windows 10 pro on it and I need a product key to activate windows 10 pro.

it says I have a "Current Product key: *****-#####".

Looks similar to this:

 "Product Key: XXXXX-XXXX-XXXXX-XXXXX-XXXXX".

I've never seen this product key anywhere. only the one sheet of paper that's was in the box was accidently thrown away before I could see what it was.