External rotation at the glenohumeral joint is vital for a successful front squat for two simple reasons. First, externally rotating the shoulders allows you to bring your hands CLOSER to the bar. Therefore if shoulder flexion continues to be limited and your hand can't reach the bar, you can get some extra range by 'cheating' and externally rotating your shoulders. Second, in terms of #PREHAB, external rotation of the glenohumeral joint opens up the subacromial space. This is the space that is commonly narrowed in shoulder impingement, so increasing your shoulder external rotation range of motion will allow for more clearance in the subacromial space and less chance for impingement.
To perform, simply use a dowel or stick as demonstrated. Place the stick BEHIND your elbow. Relax your shoulder and with your other arm, PULL the bottom of the stick towards midline. What's best about this particular stretch too is that you're gaining range in the exact same shoulder position necessary for the front squat (-90 degrees of shoulder flexion). Shout out to @unchainedphysio for this one!