It's a Dell. It's the F12 button, but you have to time it just right - or not... You should be able to just press it rapidly a few dozen times until you hear it beep.
Reboot the computer and press F12 over and over again as soon as you reboot it. Have a finger already pressing up and down on the F12 key when you reboot it. Dell is always F12, so far as I know.
Also, UNetBootIn has had some issues with recent versions. So, if it doesn't work, use Balena Etcher to write the image to USB.
Finally, your device may be so old that it doesn't boot to USB. It may require optical media to boot (or an internal drive, of course).