: When you reinstall Windows 8.1, the installer should automatically detect this key and activate the system without prompting you to enter one.
If you are trying to activate Windows 8.1 on a computer that originally came with it, you likely do not need to buy a new key. where to buy windows 8.1 pro product key
If you need a new license for a different machine, your best options are established third-party software resellers. Where to buy a windows 8.1 product key - Microsoft Learn : When you reinstall Windows 8
: Most laptops and desktop PCs from major manufacturers (like HP, Dell, or Acer) that shipped with Windows 8 or 8.1 have the product key embedded in the motherboard's BIOS/UEFI. : When you reinstall Windows 8.1