Created attachment 116498 [details]
Compare how it works in window rules and in shortcuts settings

SUMMARY
In Custom Shortcuts / Window action you can define window trigger. It that window there is Autodetect button, but it does not work correctly. See attached video.

STEPS TO REPRODUCE
1. Go to System Settings/Shortcuts/Custom Shortcuts/
2. Add New -> Window action -> Send kyyboard input
3. Go to Trigger or Action tab, press New button 
4. A window will appear, where you can define window properties which you want
5. There is Autodetect button there. Press it. Mouse cursor shape becomes an aim.

OBSERVED RESULT
When you move your mouse outside system settings window, mouse cursor becomes usual and you cannot detect window properties by clicking to some window.
When you move mouse back over system settings, it again becomes an aim, but you cannot detect properties of system settings window either.
You cannot cancel detection by pressing escape. The only way you can get out of this mode is closing system settings using keyboard or close button of its indow.

EXPECTED RESULT
Mouse cursor should stay aim all time until you press escape or click at any point. Then window properties window shoud appear.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.14.3
Qt Version: 5.11.2
KDE Frameworks Version: 5.52.0

ADDITIONAL INFORMATION
There is such function in System Settings/Window Management/Window Rules/. In a rule editing window there is a button called Detect Window Properties. It works as expected.