Good question! It depends on your machine and BIOS. I have two that can be booted by a flash drive.
The desktop has Phoenix BIOS, and to set it up to boot the pen drive, you must plug it in before boot. Then, power on and go to BIOS setup. In my system, the Lexar flash drive is considered a HDD. So, I go to the HDD list and move it to the top. Then I check the boot order, and HDD is right after CD ROM. That will boot the flash memory drive.
In my laptop, it use the Lenovo "Blue" button then F12 which lets me select a temporary boot device.
In both cases, the pen drive is ID'd by its mfgr name (LEXAR) and then HDD. Hope this helps.