<Non-US#> as your key and see if that works.
Oh. That made sense only just now. I see. Switching to Danish splits the way the OS interprets the keys... But it's "the same key" to the OS in en-US layout?
Time for me to do some reading. Trying to troubleshoot something I don't own or have access to. But it seems an interesting challenge.
So reading up on ISO layout: https://deskthority.net/wiki/ANSI_vs_ISO
With "11 keys" between the Shift buttons, it's ISO and the docs haven't been written to consider non-ANSI layouts.
For a similar keyboard layout picture like the Twiddler docs (original source?)I would look at this as it has the proper layout.
ahem after reading over the USB docs linked above, it seems that your special key is:
Usage ID: 100
Usage ID: 0x64
Keyboard: Non-US \ and | (footnotes 3 and 6 confirm)
AT-101 position: 45
and your OS is conflating it with key ID 49 (0x31), or the key at position 29.
Referencing an earlier topic from Jul 2015 apparently that key is called
<Non-US#>, which you can insert using the Twiddler Tuner.