I just set my PCSX2 (v2.20) up in Launchbox (v13.20) and it worked pretty flawlessly. I referenced the PCSX2 command line syntax, and found this to work:
-batch -nogui -fullscreen --
The double hyphen at the end will be filled in by Launchbox to denote the ROM file including path.
The other thing is that I copy the file to the Launchbox folder structure. Maybe that's the trick? 🤷♂️
I hope that helps.