If your computer came with Windows 8 or 8.1 the activation code (product key) is stored in the BIOS. Windows will automatically read the key in the BIOS and install the correct version (Core or Pro). It will activate once you go on the internet.
If Windows asks for a product key while it’s being installed, you are using the wrong edition. You have to download the the correct edition that matches your key. If your computer came with Windows 8 Core (Home) and you downloaded Windows 8.1 Core N or Single Language, it will not activate.
The Windows 8.1 iso now accepts Windows 8 Keys. It will activate Windows 8 as long as you downloaded the correct edition of Windows 8.1
If you’re not sure what edition your computer came with, download ShowKeyPlus. It will read the key in the BIOS and show you what edition to download.
no mrjimphelps - that was only true with pre-installed Vista & Win7, which used COA stickers. TrustMe is right - Win8.x OEM keys are embedded in the BIOS and no COA stickers are used with Win8.x & Win10 anymore