Due to the privacy law, German marriage records are open after 80 years. That means, that you will not find them online or publicly available.
The only way to get the certificate is to write to the Würzburg registry Office - Standesamt . The address you may find on the Homepage of the Stadtverwaltung Würzburg. But you must be a descedant of that persons and describe that, otherwise you will get no Information.