need a 25-character product key. You simply sign in with the email address linked to your subscription at Microsoft Account Services Retail Product Keys:
A product key is a 25-character code that is used to activate and validate a Microsoft Office product. It is a unique code that is assigned to each product and is used to ensure that the software is genuine and has not been pirated.
This write-up explores what Office Mondo 2016 is, the nature of its product keys, and the critical considerations regarding its use.
If you previously purchased a license, you can often find your key in your Microsoft Account under the Services and Subscriptions page.