I have an old laptop with broken display and keyboard, how do I install qubes os so that it uses external monitor and keyboard instead?

Most external monitor guides I’ve read are for people who have at least one display working.
I can install qubes with my monitor, but when I try to boot it after installation, it just shows a “_” and black screen and does nothing. It’s not hanging because caps lock lights work, I think it is asking for password on my laptop display which is actually broken. My laptop keyboard also doesn’t work so I’m using an external keyboard and mouse.
If anyone could at least please point me in the right direction, I think I can figure out the rest.

Most probably you have an issue with GPU.
Maybe try to boot with nomodeset.
What’s your hardware?

Qubes vastly prefers a PS/2 keyboard and mouse, as they are purely human interface devices. USB devices can be “kinked” to deliver malware, or contain keyloggers, or other mischief. I’m familiar with Linux and I fought Qubes trying to make it work on an HP workstation using USB gear. I finally gave up and bought a proper keyboard/mouse.

That being said, you are going to get a lot of advice about startup parameters and handling badly behaved BIOS/EFI and GPUs. This will seem more complex than getting external USB devices, but that is the correct way to approach this problem.