You have to disable secure boot, and use "Launch CSM". This will allow you to boot like a normal PC and wont fast boot.
From windows 8, choose restart, and press the "F2" key right after you see the backlight in the keyboard go out and come back on. Its quick, so be ready for it. This will get you into the Bios. (Sounds like you may have already gotten that far from your post)
From the Bios, go under the security tab, and go to Secure Boot Menu. From there, Change secure boot to disabled. Go to the "Boot" tab, and disable "fast boot" and enable "Launch CSM". This will then allow you to boot from USB/CD.
I could not boot from a CD or USB until I changed those settings. But Windows 8 uses those settings. I only did this to flash my vbios and then re-enabled the options in the Bios as Windows 8 freaks out of Secure boot is not enabled.
Im not sure at what point you could re-enable secure boot. I dont know if it would just work after you did the initial install of Windows 8 or what. maybe somebody else can chime in on that. Again, this is just what I had to do to boot to a USB Drive/DVD. I messed with it for about 3 days pulling my hair out trying everything under the sun until I found this posted on some forum regarding another Asus laptop.
(ROG has simply become too expensive compared to the competition with same specs... 😞 )
MSI GE75 Raider 10SGS - i7 10750H - RTX 2080 Super - 32GB Ram - 1TB WD NVMe - 2TB 960 EVO - 300Hz 17inch Display
RETIRED: ALIENWARE R17 R5 - i7 8750H - GTX 1070 @ 1.9Ghz - 16GB DDR4 - NVMe 970 EVO 1TB - SSD 960 EVO 1TB
SOLD: ASUS G703VI-XH74K, RETIRED: ASUS G752VY-DH72, RETIRED: ASUS G750JH DB71 , RETIRED:ASUS G74Sx DH72