@beard
You just download pfsense and install it from a USB stick. You then have pfsense, just as you would with Netgate gear. There's no need for any boot command. Just turn it on and go. When you install from the USB stick, press F11 to cause it to boot from it. Also, you can use the Del key to get into the BIOS, where you may want to change the power restore setting. The choices are off, on and previous state. Since mine is on a UPS, with APCUSD running, I picked on.
There are different sources for the Qotom. I got mine from AliExpress and it was shipped from Hong Kong. It's also available under other names. It's also cheaper than the SG-3100, but provides better performance.