As of August 2016, with the Windows 10 Anniversary Update, Microsoft allows you to link your product key to a Microsoft account. Previously, this option did not exist.
What is a Microsoft account?
This is an email address and password that you use when you use Outlook.com, Hotmail, Office, Skype, OneDrive, Xbox and Windows.
How to link a Windows 10 product key to a Microsoft account?
During the installation of Windows 10 you will be asked to enter the product key.
Once entered, at another time during installation, we will be asked to sign in with a Microsoft account and this is when the license or product key will be linked. This way, if we formatted or made any relevant hardware changes to our computer where we had Windows 10 installed, by simply signing in during the new installation, our Windows 10 will be activated without having to enter the product key.
If we are not found during the installation of Windows 10 and we already have it installed, you must activate Windows with the product key by going to Start > Settings > Activation and there click on "Change product key".
Then under Start > Settings > Accounts we sign in with a Microsoft account instead.
After all this if we go to Start > Settings > Activation we should see that it has been linked: "Windows is activated with a digital license linked to your Microsoft account"
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