OEM Windows 8 computers do not ship with product keys. The license is embedded into BIOS.
You should be able to activate it anyway.
1: Open CMD as administrator
2: Type slui 4 and press enter
3: Follow to on-screen instructions.
You can make the phone call for free with Skype.
Good luck, keep us posted.
I was not aware that the license is in the BIOS, is this done automatically when the OS is installed if it is a self-built PC?