Continuing the discussion from Are they being serious with this?: I think they must have changed it as the start button is now missing I’ll show you how it use to work though and file a bug report because I really enjoyed this feature to see how things worked.
Steps:
Move to Select mode
Go to My brawls
A number of the modes work but go to Missions vs Survivors
Select (Any Vehicles)
Add a few AI players
Hover over your own name and pull up the context menu (right click on PC and select move to spectator)
Refill your spot with an AI
Set up the rest of the custom battle like map and the other settings.
It would look like this before you started with the start button next to the leave button:
I haven’t done this in a few versions but it was nice to be able to do.