With UEFI, the key is embedded in the bios for the OEM install, unless you want to install a different version on the machine, then what came pre-installed. Then you need to use that key that came with the OS. Also you may have to go into the bios and turn off "Secure Boot".
@ broe23 Um, key as in keyboard key not key as in product key.
@ forwinhelp I've seen that option missing from the trouble shoot repair menu even on a UEFI install. It's happened to me twice now. I installed 8.1 Enterprise on my laptop and did a UEFI install. EFI partition was there but no Boot to UEFI option in the menu. I installed 8.1 Pro on the same laptop using the same procedure and its there and usable. I installed 8.1 Pro on my wife's laptop, EFI partition is there but boot to UEFI option is missing. I can still get access via pressing the right keyboard key during boot up. You may have to turn off Windows 8.1's fast startup though. http://www.eightforums.com/tutorials/6320-fast-startup-turn-off-windows-8-a.html