Start grub on external monitor (as laptop monitor unresponsive/broken)

How can I have grub start by default on an external monitor? My problem is that my laptop screen is broken/unresponsive, and I cannot see grub, as the external monitor is started only once grub has selected Ubuntu! I can hear the grub beep, count the 5 seconds timeout, but I never am able to access the grub menu?

Unfortunately, the method in this this post changing defaults in /etc/default/grub does not seem to work:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=LVDS-1:d"

and I replaced LVDS-1:d with DP-1-2-2, from:

$ xrandr -q
eDP-1-1 connected
DP-1-2 disconnected 
DP-1-2-2 connected
DP-1-2-3 connected primary

But this does not work, I don't see grub, and the monitor only starts once Ubuntu has been selected. I can think of 3 explanations:

1. I am not specifying the screen the right way? What is the d in LVDS-1:d?
2. I use actually encryption, not sure if that changes anything
3. Laptop is connected to a dock, which handles the dp ports

Any idea? Thanks!!

Thanks!