As far as I remember, the default for Multi_key on X11 is Shift+AltGr, which you should be able to add to the keymap as LSHIFT(Key_RightAlt). If that does not work (and there’s a chance it won’t, because AltGr+Shift is typically another binding, so order matters, which means you need to send these separately.), you can try a macro like this:
We should really add a page (or three) to the Kaleidoscope wiki with instructions for determining what the correct Key value to put in the keymap that corresponds to an existing key on another keyboard. In other words, how to translate the output of xev on Linux, Karabiner Event Viewer on macOS, and…whatever can do this on Windows.