Changing this setting will cause AM to automatically run the last played game when launched.
Settings-->General-->Start Up Mode = Launch Last Game
With this setting you can simply play your desired launch game before exiting AM each time and it will run the next time you launch AM.
The attract.am contains the information of the last played game. So you could run AM and play the game you want to run on launch and then exit AM and change the attract.am file to "read only". This should give you what you want but might have side effects, you'll have to try and see.
Another approach might be to create a batch file that launches your desired game without AM from the command line and then runs AM when you exit the game. There's also the option to run a script on AM launch or exit that will modify the attract.am file to contain the correct game to run when launched.
