https://www.reddit.com/r/GetComputerHelp/comments/y17yo3/where_to_get_a_windows_10_key/
What is the product key of Windows 10?

The product key to activate Windows 10 is not an easy to locate. To activate your version of Windows 10 it is necessary to enter the code associated with your device's serial number. This code is located on the sticker or label that you used when your device was upgraded or purchased. If you can't find the sticker, Microsoft offers a download tool called Activation Troubleshooter that will assist you in entering the key to activate Windows 10 onto its activation page.