If the belt is squealing due to moisture on the belt which could be causing the belt to slip on the pulleys then yes spraying some WD 40 on the belt can help stop the squealing or squeaky belt.
However if the belt is squealing or squeaking as a result of the belt being stretched or because the tension device or tensioner is not working properly then the WD 40 won't fix the squealing or squeaking belt.
Make sure the tensioner is keeping the belt tight and that the belt is in good shape and not stretched.
Besides moisture on the belt a stretched belt due to age or a bad belt tensioner can be a cause of a belt squeaking or squealing.
The belt needs to be properly tightened and at the proper tension to move on the pulleys properly without slipping.
If the belt is loose on the pulleys and slipping then the belt could squeak and squeal and could eventually come off the pulleys.