Psalm 110:4 says the one addressed is a priest forever in the order of whom?

Answer: Melchizedek

"The LORD has sworn and will not change His mind: 'You are a priest forever in the order of Melchizedek.'"

“The LORD has sworn and will not change His mind: “You are a priest forever in the order of Melchizedek.””— Psalm 110:4 (BSB)