The product key is embedded in the UEFI BIOS it will automatically be input with Windows 8.1 (with update 2 media) or Windows 10 TH2 installation media. Follow the instructions here to download and create a Bootable USB (for UEFI BIOS): [URL] [URL] 
I need to replace the hard drive on my Inspiron 13-5323.  It came with the factory "injected" Windows 8 on the motherboard. I subsequently upgraded to Windows 8.1, then Windows 10.  With no Windows disk and no product key, how can I reinstall Windows onto the new hard drive? Illinichip
Reinstalling Windows 8 (or 10) after replacing hard drive