It would seem to be a simple request. But, not so. There’s even a bug.
When I try to assign the shortcut, it showed “Launch (1)” as the assigned key.

I was able to resolve it using the tips at
https://bbs.archlinux.org/viewtopic.php?pid=837944 which suggested:

I searched for the string “Launch (1)” recursively in my directory ~/.kde and
found it in the files

~/.kde/share/config/kglobalshortcutsrc
~/.kde/share/config/khotkeysrc

In both files I had to replace “Launch (1)” with “Calculator”. Oddly enough I
had to replace it with “Calculator”, though the terminal application xev
actually indicates the name “XF86Calculator” for that key.

Afterwards you have to reboot, then your special calculator key should work.

After changing my files to be ‘Calculator’ instead of ‘Launch (1)’ it worked too. Not very elegant. Hope it’s fixed soon.