You have to first get called up to the major leagues and once you do, hit a home run in any game. Don't worry if you don't get a postgame interview right away after the game, such as if you use the Play Next Game feature and go straight into the next game in the series. When you finally exit to the Clubhouse at the end of the series, you'll get interviewed about hitting your first home run and the trophy will pop after concluding that interview.