Go to your schedule, click on 'Exhibition Challenge'.
Choose a team to challenge from the list. If it is a Sun AI team, they will automatically accept and the game will be added to your schedule.
If the team is coached by another user, then they'll see a 'pending challenge' - which they can either accept or reject.
You may schedule up to 2 exhibition games per season. Games must be scheduled prior to 1 PM ET to be played that day. Once a game is scheduled, it can not be cancelled.