Well first things first. Very few keyboards support Wayland and if I remember correctly even then. There needs to be some work on the compositor side for third party ones to work.
The one that I know for sure works with Plasma is the maliit keyboard. The package you need to install is called maliit-keyboard.
From there open up system settings → go down and click on the keyboard section → then click on virtual keyboard. That will get you to the screen in the screenshot you provided and from there you should be able to select the maliit keyboard.