This achievement is no longer available. Original solution:
Unlocks after you completed all online cards. Just try to play as many games as possible with your team and perform well. You can also check your requirements for each Hockey League rank/card from "Main Menu > Performance tracker," and by pressing RB to switch from offline to online ranks.
The requirement you need to get the online Legend Card is an A- and 150 games. The previous cards will unlock as long as you play games and maintain at least an A-.