When installing Windows 10, you will see a prompt: "Enter a product key." Look for the small blue text at the bottom:
You can activate Windows 10 without a product key by using the Command Prompt and a generic product key provided by Microsoft for installation purposes. how do i activate windows 10 without a product key
If you later buy a legitimate key, activate it: When installing Windows 10, you will see a
: If you have a valid, genuine key from an older version of Windows, you can often enter that key in the Activation settings to activate Windows 10 for free. Using Windows 10 Unactivated When installing Windows 10