How can I force my computer to use the DisplayPort?

My computer, an old Lenovo Thinkcentre, sometimes doesn't display when it boots. I guessed that was because it was trying to use its VGA port instead, so I bought a VGA cable. I had to turn my computer off today, and when I turned it back on the display was blank - until I plugged in the VGA cable. (The monitor has jacks for VGA and HDMI; I have a DisplayPort to HDMI cable.)

How can I force it to use DisplayPort? I can't find anything in the BIOS setup that looks likely.