The following steps outline how to start a program or script automatically when launching KDE Plasma 5. Luckily, Plasma has an Autostart Manager module that makes this very simple.

First, search for autostart and click the result to launch the Autostart Manager

KDE Plasma Find Autostart Manager

KDE Plasma Autostart Manager

Click Add Program... and search for the program you would like to autostart and press enter. Alternatively, you can navigate the program tree.

KDE Plasma - Autostart Manager - Add Program

Confirm the program by pressing OK. You'll then be able to see your program added to the list. Hit OK.

KDE Plasma Autostart Manager - Confirm Program

KDE Plasma Autostart Manager - Program Added

Upon restart, your program should now automatically launch along with Plasma!