Go to your schedule, click on 'Schedule Exhibition'. Choose a team to challenge from the list. If it is an AI team, the game will automatically 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.