This is just a quick reference for what worked for me and is by no means a complete guide. It seems there are lots of reasons why it might not work but here’s mine.

Virtualbox

First configure virtualbox display settings’ graphics controller to be vBoxSVGA.

In my case, before I had tried this, adding the new mode and switching to it would result in a completely black screen of the correct resolution.

Commands

First run

cvt 3440 1440

Where 3440 and 1440 are replaced by the desired resolution. This should generate something like:

Modeline "3440x1440_60.00" 419.50 3440 3696 4064 4688 1440 1443 1453 1493 -hsync +vsync

You’ll need to copy everything after Modeline and place it as the arguments to the xrandr --newmode command as such:

xrandr --newmode "3440x1440_60.00" 419.50 3440 3696 4064 4688 1440 1443 1453 1493 -hsync +vsync

From here you can add the created mode to a display.

xrandr --addmode VGA-1 "3440x1440_60.00"

In my case the display Virtualbox has came up as VGA-1 but you’ll have to check your own by running xrandr.

Automagic

I’d strongly recommend adding this to a startup script for when the computer boots.