electrum-4.4.2-x86_64.AppImage : commande introuvable
Since you're running an appimage instead of a command, you'll need the "./" at the start.
If it didn't worked, enter this command first before launching it:
And most importantly, the portable flag should in uppercase -P; the lowercase -p is for --proxy.
I never knew this was possible for any AppImage, to be honest. But it's not like I'm going to run --help on the binary to see a list of options, because I know they're all bundled in the same way.
If you ask me, the Linux desktop environments do allow you to click and run an AppImage, but they should also allow you to customize options like these in their Properties panels. Currently I don't know of any DE that implements this.