If you activated a free upgrade to Windows 10 or bought and activated Windows 10 from the Microsoft Store, you'll have a digital license for your device instead of a product key. This means you can reinstall the same edition of Windows 10 that your device has a digital license for without entering a product key.
During reinstallation, if you’re asked to enter a product key, select Skip. In most cases, if Windows 10 was activated on your device before you reinstall, it will automatically activate after you go online.
More info here.
https://support.microsoft.com/en-us/help/10749
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