There is no direct connection from Supetar to Milna. However, you can take the line 631 ferry to Split then take the ferry to Milna. Alternatively, you can travel from Supetar to Milna in around 20 min.
From Supetar line 631 ferry to Split then ferry to Milna. 2 h 3 min 44 kn. From Supetar taxi to Milna. 20 min 251 kn. From Supetar drive to Milna. 20 min 17 kn. From Supetar Uber to Milna. 20 min 110 kn.