
If you are prompted to enter a product key, click Do this later and give it a few days, Windows 10 will activate automatically.If you perform a clean install on the first attempt, you will have to start over from an activated Windows 7 or Windows 8.x installation.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.

The most notable being that Windows 10 is free for existing Windows 7, 8/8.1 users for the first year.
