On Linux Mint Cinnamon, I can do something like this:

In a terminal:

sudo apt install putty

to install a program that can handle the telnet URL.

Then, create the file telnet.desktop in "/home/yourusername/.local/share/applications" with the following content:

[Desktop Entry] Name=Telnet Handler Exec=putty %u Icon=telnet-icon Type=Application Terminal=false MimeType=x-scheme-handler/telnet;

Then, in a terminal again, run:

xdg-mime default telnet.desktop x-scheme-handler/telnet

Then, when clicking on a telnet URL in Opera, it'll ask you if you want to open it with xdg-open (or maybe kde-open for you) and when you do, it'll launch a putty terminal based on the telnet URL.

To get it to launch with the telnet program instead, I'd probably have to set Exec to a bash script that parses the telnet URL and launches ```telnet "the site address" "port". The script would probably have some protections and error handling of the URL etc. too.

Anyway, not complete, but perhaps something like that will work for you in KDE and get you started in the right direction if you can't find anything in the KDE settings.