The Windows 8 product key is actually embedded in BIOS and was not provided in a physical form with Windows 8 units. This was to remedy the problem of the COA sticker becoming illegible or losing the COA sticker altogether.
If the version of Windows 8 being installed is the same version the unit came with (e.g. Installing Windows 8 Home Premium 64bit on a system that came with Windows 8 Home Premium 64bit), then the installer will read the product key from BIOS and proceed as usual.