I tried a few times with different usb sticks , I used Rufus, Disk imager but nothing happened...... not even try.... the computer freeze. Then I tried with a cd but it said something about missing Kernel.
To be sure the usb boot was working I have a windows 10 on a usb flash drive and the windows 10 OS booted a run the installation with no problem.
I'm going to try now with a different Usb stick 2.0 (I downloaded FreeNAS again, I used Etcher this time to write it to the usb stick) and I'm going to use the MB usb port in the back of the computer. I'll let you now if I'm lucky this time.