Don't you have your license number in BIOS?

UEFI = latest BIOS technology ~10 years old ( born out of EFI )= CSM disabled
BIOS = technology that was invented ~40 years ago! = CSM enabled ( in G751 bios menus )
they're are both do what the BIOS abbr. stands for: basic I/O system - a "basic" software that communicate with the different HW connected to form a PC.

About the "serial" been "in the bios"... it is more coplicated then this, as it has a lot with some special table that the OEM ( ASUS ) has installed inside the "bios" itself... it is called "SLIC", and this "tech" has been advanced since win 7, so it isn't possible anymore to just "flash" a mod bios with "SLIC" with the right info...
moreover, once your laptop ( once that came with pre-installed windows 8/8.1/10) connected to the net, MS servers got your unique "HW id", and from that moment on, you can transform any cleaned installed windows ( of the same version ) to be a genuine windows, just by connecting again to the internet ( the activation process being done automatically once you enter the "activation" screen by right click "computer"->properties ).
Moreover, once MS server got your "HW ID" & you're activated, you can activate your any future installation of same window version with a general "product key" - a number that is freely and officially published by MS ( if there is some "traffics" with the auto-activation ).
Please note: you MUST install the SAME OS version ( if you had win8.1 HOME install that one not PRO version!)
and you MUST install it with the "OEM" channel.
To actually know how to perform that installation - please refer to the sticky thread "How to install windows 8/8.1/10 The proper way (UEFI based installation)"