protocol PassDataDelegate {
    func passData(data: String)
}

class FirstViewController: UIViewController {

    @IBOutlet weak var textField: UITextField!

    var delegate: PassDataDelegate?

    override func viewDidLoad() {
        super.viewDidLoad()
        delegate = SecondViewController()
    }

    @IBAction func sendDataButtonTapped(_ sender: Any) {
        delegate?.passData(data: textField.text!)
        performSegue(withIdentifier: "Go", sender: nil)
    }

}

class SecondViewController: UIViewController, PassDataDelegate {

    @IBOutlet weak var myLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    func passData(data: String) {
        print("This came from first: (data). Will change UI.")
        myLabel.text = data
    }
}

delegate?.passData(data: textField.text ?? "")