This command works:
flatpak run org.freefilesync.FreeFileSync "/home/username/realtest.ffs_batch"
flatpak run --command="RealTimeSync" org.freefilesync.FreeFileSync "/home/username/realtest.ffs_batch"
flatpak run org.freefilesync.FreeFileSync "/home/username/realtest.ffs_batch"
flatpak run --command="RealTimeSync" org.freefilesync.FreeFileSync "/home/username/realtest.ffs_batch"