Activate Windows - Microsoft Support
https://support.microsoft.com/en-us/windows/activate-windows-c39005d4-95ee-b91e-b399-2820fda32227
WEBActivate Windows using a product key. A product key is a 25-character code, that looks like this: PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX. During installation, you'll be prompted to enter a product key. Or, after installation, to enter the product key, select the Start button, and then select Settings > System > Activation > Update product ...
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