class TestClass {
    @objc static func testFunction() {
        print("testFunction")
    }
}

// MARK: - Назначаете:

yourButton.addTarget(TestClass.self, action: #selector(TestClass.testFunction), for: .touchUpInside)

class CustomButton: UIButton {
    override func awakeFromNib() {
        super.awakeFromNib()
        addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
    }

    @objc func buttonAction() {
        print("Test")
    }
}