Legend Challenge teams appear randomly during Career Mode. Each time they appear, you have 1 hour to play against them once.
In the Legend Challenge page, you see a timer counting down on the bottom left corner which indicates the remaining time you have to play with the Legend Team. Tap PLAY! to play and Quit to give up the chance to play with the Legend Team.
No matter if you play with or quit the Legend Challenge, they will come to challenge you in the future randomly.