Where did Absalom have himself proclaimed king?

Answer: Hebron

Claiming he had a vow to fulfill, Absalom went to Hebron and sent word: at the sound of the horn, say, "Absalom reigns in Hebron!"

“After four years had passed, Absalom said to the king, “Please let me go to Hebron to fulfill a vow I have made to the LORD.”— 2 Samuel 15:7 (BSB)