To set the password is quite simple, just follow these steps:
1. Go into the custom command lines by clicking the button on the control panel
2. Press new, then give the command line a name in the description box
3. In the private password box type in what password you want to use to join
4. Make sure to tick the tick box to the left of the box
5. Press save, and you will see the command line, on the right there is a select button make sure to click it to activate the command line.