If you downloaded Windows 8, chances are the product key is in your purchase confirmation email. If you purchased a Windows 8 DVD in a retail store or online, your product key should have been included alongside your disc.
If Windows 8 came preinstalled on your computer, and you are now performing a clean install of Windows 8 on that same computer, your product key is probably located on a sticker located somewhere on your computer or device.
Once you've entered the product key, click or touch Next.
Important: Entering your product key at this point in the Windows 8 clean install process is not optional. This is unlike in previous versions of Windows where you could skip the product key entry during installation as long as you provided one within a certain time frame, usually 30 or 60 days. Also unlike in pervious versions, activating your Windows 8 product key online is automatic and part of this process.
Tip: As I mentioned in the first step in this tutorial, if you've lost your product key, and you're performing a clean install of Windows 8 over an existing, and working, retail copy of Windows 8, then you should be able to extract the valid product key you used to install Windows 8 the last time. See How To Find Your Windows 8 Product Key for help.