change the following lines (as below) in the .plr file that the dedicated server uses (depending on how your server is set up it might be the player called "PlayerNotUsed")...
Closed MP Qualify Sessions="0" // if true, the session will be closed during
qualify, so as to not disturb the participants
Skip Formation Lap="1"
MULTI Formation Lap="0"
Extra Stats="1" // Saves separate verbose stats for any session (instead of
simple stats for most recent race only)
Timescale Ded Sessions="0" // Dedicated server sessions get timescaled with
race length
Lightning Probability="0.00000" // Probability of seeing lightning (0.0-1.5)
Thunder Probability="0.00000" // Probability of hearing thunder (0.0-1.5)
I've seen it
suggested that you should also set the following, or you risk a full yellow being thrown because of stationary grid, but I've seen no evidence for this.
MULTI Full Course Yellows="0"
Also look at
Aagramn's post below.
Note that you don't need any of this unless you're hosting a game.