I would assume you sign up at the Universität für Musik und darstellende Künste Wien.

A Master in Sacred Music is the degree expected for Cathedral organists and other high-level Church Musicians in Germany and Austria; think of Churches with a well known choir, such as the Windsbacher Knabenchor, or Dekanatskirchenmusiker (a Church Musician responsible for a deanery, e.g. for the formation of part-time Church Musicians).