Where did Balak send messengers to summon Balaam son of Beor?

Answer: Pethor

Balak sent for Balaam at Pethor, "by the Euphrates in the land of his people" (KJV: "by the river").

“he sent messengers to summon Balaam son of Beor at Pethor, which is by the Euphrates in the land of his people. “Behold, a people has come out of Egypt,” said Balak. “They cover the face of the land and have settled next to me.”— Numbers 22:5 (BSB)