There is no CSM. BIOS is UEFI only.
Turn off Secure Boot in BIOS.
Insert bootable USB into keyboard slot, restart.
When you see camera light flash, keep tapping ESC key which will bring up boot device selection menu, select the one that looks like it's likely the USB. Contrary to all the other help, holding key down does not work.
What are the Boot Option Keys for T100?
ESC: Boot device selection menu
DEL: Boot into BIOS Setup (same as F2)
F2: Boot into BIOS Setup
F9: Boot into windows recovery mode
Vol Down: Boot Device Selection Menu
Asus T100 Master Help Guide
BTW, Windows 8.1 with Bing is what was installed. Standard 8.1 won't install. Both are 32-bit.
There is no official MS download for 8.1 w/Bing, but I just found an iso download by Googling for "windows 8.1 with bing download iso free". Legal for you to use on the T100, no PID required, it's tied to the UEFI security (TPM) chip.