Connect an external monitor to laptop with broken screen to display GRUB menu

I'm on Lubuntu and I'm not familiar with what monitor layout or autostart packages/tools ship in Ubuntu. I suspect that Ubuntu has similar tools, if not the same ones as these. I hope this gives at least a place to start working.

On Lubuntu, I use Menu -> Preferences -> Default Applications for LXSession, Autostart to run commands and scripts that adjust mouse settings + the like. It will probably run an xrandr command to set monitor configuration as well.

The easiest way to generate the command to set your desired monitor configuration is with ARANDR. ARANDR is a simple graphical user interface to xrandr (X Resize and Rotate monitors). If this isn't installed, it's available in Synaptic. (You can also figure out the command without ARANDR. It's not particularly complicated but may take more time - look up xrandr manual.)

Start ARANDR and click Outputs in the menu bar. Activate + configure them how you want. Probably something like LVDS off (LVDS= local video display screen), and VGA-1 or HDMI-1 on (however your external output display screen is connected). Then save that layout with Layout -> Save As. Make a note of where you save it, or just put it on your desktop for convenience.

Then open that file with Vim, Leafpad, or another text editor. Copy the command that starts with xrandr --output. Be sure to copy the WHOLE command, not just those first 2 words, or the first line. Close that file, without making any changes.

Then from the Menu, select Preferences -> Default Applications for LXSession and select the Autostart section (button on the left). On the right, look under the heading Manual autostarted applications for a button labeled + Add, and paste the whole xrandr --output ... command from the file into that box. Click the + Add button. That command will then be run the next time you boot.