With this trick you can launch the desktop mode as custom Steam game and you can use the GameScope tools in desktop mode, to setup the refreshrate TDP or something elese.
At first you have to create a script:
#!/bin/sh unset LD_PRELOAD exec startplasma-wayland --xwayland --x11-display $DISPLAY --no-lockscreen --width 1280 --height 800 -- plasma_session
Than you must make it executable and now you can add it as custom Steam application.
Thanks to u/seba_dos1 for the trick.