In many cases, you won’t need to enter a product key to install Office. Just sign in to your account page and click Install. If you're prompted to enter a product key, you can find it in one of these places:
If you downloaded Office from an online store, the product key should be in your email receipt.
If you didn’t get a receipt, check your junk mail folder.
If Office came with a box, card, or disc, look on the card or the packaging.
If Office came with your computer, look on the Certificate of Authenticity.