By default, Windows 10 builds are pre-keyed, meaning, you do not have to enter a product key and should not be prompted to enter one, as long as you upgraded from an activated Windows 7 or Windows 8 or Windows 8.1 license or Windows 10 Preview build.Windows 10 uses its own unique product key.
Understand and Fix Windows 10 Product Key Activation For more on activation issues, take a look at these articles: Update: This was written for issues when Windows 10 was initially released in 2015. Regardless of this, product activation remains a part of Windows 10. In this article, we take a look at product activation and troubleshooting steps.
The most notable being that Windows 10 is free for existing Windows 7, 8/8.1 users for the first year.