This is what helped me:
Identify the external port. In my machine this is DP-1. xrandr lists them, but they have slightly different names for the kernel. For instance, xrandr calls DP1 what the kernel calls DP-1.
Boot the machine with the port enabled, by passing the parameter
e means "force output to be enabled, even if it looks like there's nothing on the other end".
For instance, I have to add:
To boot the kernel passing a parameter: enter the grub menu at boot (press Shift if the menu is skipped), press
e on the first menu entry (default, latest kernel), go to the line that begins with
linux /vmlinuz or something similar, and add your choice of
video=<port>:<res>e to the end of that line. Then press Ctrl-X to boot.
The resolution, as far as I know, is mostly irrelevant, although the ratio seems to be. xrandr/Ubuntu will later let you play with it a little and change it.
When you boot, you should be able to see "a monitor", although it may appear disabled by default. Try to enable it a few times. In my machine, it doesn't always work the first time, but it may work the second or the third without reboot. (Why more than once? Because it's not Haskell :S I don't really know.)