Posted in order to give the answer (QnA).
I have installed Lxqt desktop on top of Kubuntu 18.04. That means I can still use the
krunner launcher of KDE/Plasma if I chose to, but Lxqt has its own launcher called "runner" (lxqt-runner) that is similar and very light.
It is supposed to be used with Alt-F1. That can be changed in the Shortcut Key settings but not to just the Super-key.
Is it possible to make this launcher run with only the Super key?