You need to create an "a2600.ini" file with the following lines:
# SLOT DEVICES
# IMAGE DEVICES
cartridge "enduro (1983) (activision, larry miller) (ax-026, ax-026-04).bin"
The first (not commented) line is what you are looking for. The last line it's for showing you can load an image right at the start. In case you ask how I generated the INI file I did a "mame64.exe a2600 -cc" command line, renamed the created "mame.ini" to "a2600.ini" and deleted everything else above "# SLOT DEVICES". Make sure to create a backup first so you don't lose your settings.