You can password protect your server by doing the following:
1. Stop server.
2. Click "Configuration Files"
3. Click "Text-Editor" next to Saved\Config\WindowsServer\ServerSettings.ini
4. Find the line: JoinPassword=
5. Type your desired password after the = so it will look similar to this JoinPassword=MyServerPassword123
6. Click "Save and Exit" on the navigation bar.
7. Start your server.
