I had a problem getting the boot menu and the system menu to appear while rebooting a Dell Optiplex 9020.
According to the manual here, the System menu in BIOS comes up with the F2 key and the boot menu comes up with the F12 key. But when I plugged a USB keyboard in, I could not get those options to appear before it booted Windows.
I noticed mini DIN keyboard and mouse connectors on the back; Why would a brand new computer have a really old interface? So, I found a mini-DIN keyboard and plugged it in. It works, now you can get the data to the BIOS before the DOS / Windows takes over.
My diagnosis is that the BIOS does not initialize the USB device in the proper sequence early enough so that you can use the keyboard in time, perhaps that's why they included mini-DIN interfaces? Great design feature, Dell!