SUMMARY
In the popup that allows you to add a property to a window rule, there is a searchbox that allows you to filter the list of properties. It would be convenient to have the focus in this textbox when the popup is opened. 

STEPS TO REPRODUCE
1. System Settings > Window management > Window rules
2. Edit any existing rule
3. Click 'add property'

OBSERVED RESULT
A popup opens with a list of properties to add. There is a textbox at the bottom to filter the properties, but the focus is initially not in this textbox.

EXPECTED RESULT
When the popup is opened, focus should initially be in this textbox, so that users can at least type to filter without having to click and place the focus in the textbox.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.21.0
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2