How to set up `<Primary><Alt><Shift>semicolon` as a shortcut key in xfce4-keyboard-shortcuts?

Question

I'm trying to duplicate Google Sheets' shortcut to insert current date and time stamp under Xubuntu.

I think I'm almost there as I have a shell script ~/bin/clipdate.sh

#!/bin/bash
date +'[%Y.%m.%d-%H.%M]' | xclip -sel clip

and I've set /commands/custom/<Primary><Alt>semicolon as Property and /home/****/bin/clipdate.sh as Value. And it works, I hit the keys, and the timestamp lands in my clipboard nicely.

But I want the exact same keys :/ trying:

• /commands/custom/<Primary><Alt><Shift>colon doesn't work
• /commands/custom/<Primary><Alt>semicolon doesn't work

and I have another shortcut consisting of <Primary><Alt><Shift> and another key (L if that matters) and it does work so it seems it's not a question of using all three.

Is this some sort of a bug, am I doing it wrong or is it just not possible for some reason?