How to use a Steam Workshop map
Step 1: Locate the map that you would like to use located in the Steam Workshop and subscribe to it.
Step 2: Once you have selected the map that you would like to use, at the top in the URL for that mod, locate the ID of the mod. For instance, City Survival would be 674794464
Step 4: If you have not created a commandline, you would now create you're commandline.
Step 5: In the map section of your commandline, you would put w-mapid. For example, City Survival would be w-674794464.
Step 6: Save your commandline.
Step 7: Start your server and the map files will download.