Вот оно: просто вставьте код ниже в ваш метод viewDidLoad.
let viewLeftButton: UIView = UIView(frame: CGRect(x: 0, y: 0, width: 160.0, height: 32.0))
let textField: UITextField = UITextField(frame: CGRect(x: 0, y: 0, width: 100.0, height: viewLeftButton.frame.size.height))
textField.borderStyle = .roundedRect
textField.font = UIFont.systemFont(ofSize: 14.0)
textField.placeholder = "Type here..."
let button: UIButton = UIButton(frame: CGRect(x: textField.frame.size.width + 8, y: 0, width: 44.0, height: viewLeftButton.frame.size.height))
button.setTitle("Click", for: .normal)
button.titleLabel?.font = UIFont.systemFont(ofSize: 14.0)
button.setTitleColor(.blue, for: .normal)
viewLeftButton.addSubview(textField)
viewLeftButton.addSubview(button)
self.navigationItem.hidesBackButton = true
let leftBarButton = UIBarButtonItem(customView: viewLeftButton)
self.navigationItem.leftBarButtonItem = leftBarButton