You are P1, the Forehand. All four passed, and you play the Call.
P4Dealer
Your hand
Which King do you call?
Answer
The King of Hearts. You hold no Heart: when Hearts are led, you play a Tarok, and its holder is on your side. You hold three Diamonds, so the King of Diamonds would join a suit where you follow anyway.
Rule: Call a King in a suit where you are void: when that suit is led, you play a Tarok, and the player who holds the King is on your side.