Steps:
1. Open the File Manager
2. Navigate to /server-data/Config/WindowsServer/
3. Right click Engine.ini and click Edit
4. At the bottom paste the text for the map you would like to use:
Exiled Lands
[/Script/EngineSettings.GameMapsSettings]
ServerDefaultMap=ConanSandbox
Isle of Siptah
[/Script/EngineSettings.GameMapsSettings]
ServerDefaultMap=/Game/DLC_EXT/DLC_Siptah/Maps/DLC_Isle_of_Siptah
Modded Maps
Note: You will need to replace ModdedMapLocation with the actual location of your map on the server.
[/Script/EngineSettings.GameMapsSettings]
ServerDefaultMap=ModdedMapLocation5. Close and save the file
6. Restart the server and the map will load in game.