If you have purchase the MS Office product from the retail store, then the product key will be there on the backside of the retail card. But if you have purchased this Office product key online then this product key will be there on your registered email ID. For getting the Office product key, you will have to login in your email ID and check the inbox. Do not share this product key information with anyone as it is for your use only.
https://www.office-com-setup.com/