Plug in the USB stick, then in the bios:
Integrated Devices -> make sure USB Controller is “On with BIOS Support”
USB Flash Drive Emulation Type -> should be “Hard disk”
Hard-Disk Drive Sequence -> make sure the “Hard-disk-emulated USB flash drive” is first
Should now boot into USB (if you remove the USB at some point after setting the hard drive sequence it seems to change it back to actual hard drive over USB, so if you remove it and plug it back in to boot from it again, you’ll need to go back into the BIOS to change the setting again).