Add --loadmap=your_mapname into your .bat, .sh or launcher (I have no idea how you do this on a Mac), eg.
Use a simple text editor to edit the file.
edit: oh, wait, I just read "...for instance if nobody is in the server...". My solution loads that map when you start the game. The only way to get a specific map to load in a server is if you control the maprot file for that server, and you set it as the only map that supports 1 player.
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bin_win32\ac_client.exe --loadmap=ac_awesometastic --init %1 %2 %3 %4 %5edit: oh, wait, I just read "...for instance if nobody is in the server...". My solution loads that map when you start the game. The only way to get a specific map to load in a server is if you control the maprot file for that server, and you set it as the only map that supports 1 player.