What did King Josiah do before going out to fight Pharaoh Neco at Megiddo?

Answer: Disguised himself

Ignoring Neco's warning that God was with him, Josiah disguised himself to fight him. Archers shot the king, and he died in Jerusalem.

“Josiah, however, did not turn away from him; instead, in order to engage him in battle, he disguised himself. He did not listen to Neco’s words from the mouth of God, but went to fight him on the Plain of Megiddo.”— 2 Chronicles 35:22 (BSB)