@progets
Thank You!
I see how that works now.
I had to try several cores and finally I was able to get it to work. (mame 2003 plus worked for bublbobl.zip)
For those interested:
Adding a specific Retroarch core to a rom in Attract Mode (PC):
1. Set a key or button combination to edit game
in Attract Mode -> configure(TAB) -> controls -> Edit Game. (for me EDIT GAME was set to ESC+down)
2. Highlight your game and press the edit game button/key combo
3. Select Custom Executable, enter the same as in your layout emulator settings, in my case: C:\RetroArch\retroarch.exe
4. Select Custom Argument: enter the same as in your layout emulator settings, but change the core, in my case the default in my layout was: -L \RetroArch\cores\mame2010_libretro.dll "[romfilename]"
but I wanted the mame 2003 plus core so I used this : -L \RetroArch\cores\mame2003_plus_libretro.dll "[romfilename]"
Thanks to progets for the help