You can find the config in
This how it looks for me after editing it:
The .desktop file has to be in
Exec=lxterminal -e omxplayer -b %f
It is important that the terminal is open, because once you start omxplayer without a terminal you won't be able to close it easily.
For me, using Terminal=false caused the terminal not to be focused, so I needed to switch to it first (alt + tab) to use keyboard shortcuts. That's why I'm using lxterminal -e. Make sure to use a terminal emulator that is installed on your system, it should also work with any other one that can accept commands as argument.
You can now double-click .mp4 video files and a terminal will open up that will spawn omxplayer.
%f term is the placeholder for the filename.
-b will cause the screen around the video to go black.