Normaly you would create a button or something that calls “open YOURLEVELNAME?listen” in the console.
But if you want to create a server when starting the game, you would need to add “YOURMAPNAME?listen -game” to your .exe command.
If you do this over the CMD, you would search for your .exe game file and
do something like “MyGame.exe MyMap?listen -game” and hit enter.
Or you add “MyMap?listen -game” in the settings of the exe (rightclick etc).
I have never done this, but i assume this is the way to do it.