The same as previous answers: keep it in contact with food, for prolonged time - up to 1.5 hrs. It could be container feeding, repeated showering by food every 5-10 min with flow off and on again and again, or adding cyclop eeze or other food into the water (to induce feeding responce, another options is just feed fish, if you feed a lot) and 20 min later placing mysis onto mouths. Within week it may start to open, then it will be easier.
Good luck!