The Halasz Gambit is a little back road in the Danish. After your first gambit is accepted you can proceed with 3.c3 and head for the Danish. On the other hand you can advance on the other side with 3.f4. This brings about the starting position of the Halasz Gambit.
This move allows Black to support his pawn on d4 and he usually does this by 3...Nc6. This move scores respectably for Black restricting White to a success rate of barely more than half.
His second most played move, 3...d5, is even more successful. White wins just 3 games in 10 in this line. The move appears 43 times in my database.
Many of the players playing the Halasz Gambit, which appears 219 times, are in the 2100 - 2300 range. Results from the other main replies, 3...Bc5, 3...Bb4 and 3...d6 are much less encouraging for Black.