https://www.completenoobs.com/noobs/Windows_10_Product_Key
Windows product key is a 25 character code used to activate Windows.
Find your Product Key
In the search bar, type cmd. Right-click Command Prompt and select Run as administrator.
In the command prompt, enter:
wmic path softwarelicensingservice get OA3xOriginalProductKey
Should return your 25 character product key:
OA3xOriginalProductKey
YXXXX-KXXXX-3XXXX-CXXXX-NXXXX
Make a backup, useful if reinstalling windows after distro hopping.