The thylakoid membrane contains a protein called ATP synthase. As hydrogen ions pass through the protein, ADP and a phosphate group are combined to form ATP. What is the direct energy source, if any, for the movement of hydrogen ions and the formation of ATP?

The energy source is a set of ATP molecules that gather inside the thylakoid.

The answer is; electromotive force

As the Krebs cycle / Citric cycle occurs in the mitochondria matrix, hydrogen protons are produced and get accumulated in the inter-membrane space from the matrix of the organelle. It is this electromotive force that is harnessed by the ATP synthase to produce ATP.