I don't think this exist. The Dönitz or Flensburg government existed only from May 2 to May 23, when they were arrested and the Allies took control of all government functions in Germany. While they notably signed the unconditional surrender of Germany, I do not think that any swearing-in ceremonies took place between May 2 and May 8 or that if they happened, there is footage of it or that they actually replaced Hitler with Döntiz instead of just reverting to the previous oath.

Erich Kästner in his diary Notabene: 1945: Ein Tagebuch mentions in one passage that people shortly before the end of the war did use "Heil Dönitz" in a joking manner but that seems to be the only context this ever came up.