I can't guess if your USB stick is UEFI compatible (few are) and in Windows 8.1 you get to the BIOS (or more accurately UEFI) via a special procedure like this.
I didn't find the Samsung video but this works here.
To boot USB sticks I can't have UEFI enabled for most bootable sticks. Sorry if I keep this short.
first of all i wanted to boot my laptop from usb so i disabled fast boot and secure boot and i put OS mode selection on CSM and UEFI OS.
i formatted my laptop with windows 8.1
but now i dont have access to bios setup even if press F2 several times, and theres is no UEFI firmware option on the advanced startup settings, and another time i cant boot from usb, only HDD.