Welcome to the forum! 
Unfortunately, the traditional way of typing Unicode characters by way of the Ctrl+Shift+U + hex code does not work in qt-based applications. KDE Plasma and all of its components are qt-based.
I believe there’s a workaround by way of ibus, but I’m afraid I can’t give you any more details about that.
There was however a thread about it here at the forum that I myself participated in, but that I left behind because I had been wrong — it worked on my system, but only in gtk-based applications, not in qt-based applications — and because of the rather arrogant attitude of the original poster. Yet, the thread was marked as solved later, so perhaps you will find the necessary information there to make it work.
