How do I add my own Libretro runner to Lutris?

I’m new to Linux, and I’ve been using Mint to get used to the new OS.

I’ve downloaded Lutris and RetroArch, both via flatpak, and I noticed that the RetroArch instance installed on Lutris is on version 1.9, while the most recent version of RetroArch is 1.16.

I’ve found the option to add my own executable, but I’m having a hard time finding the actual executable.

I did try copying the “exec” path by opening the desktop shortcut with a text editor, but the path provided resulted in the error shown in the screenshot.

Thanks in advance.