Yep you'd need to edit the ServerGameSettings.json file. Click the Quick FTP Settings button and you will see where to edit it in your favorite FTP client.
We'll be adding this vars to our easier to use Modify Server Config tool in the coming days.
We've added the server mode toggle to the new Modify Server Config tool. Will continue to populate the tool with more of the settings found in ServerGameSettings over the next few days.