Ios swift – How to display "add to wallet button" in flutter

I want to display add to wallet button in Flutter. I have written some native codes, channel and where to display this button. Now the button is visible. But can not control it. I can not put it to the exact place. For example I want to put a button after the texts. But the button still stays on the appbar and doesn’t move.

My AppDelegate file is like this :

import PassKit

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
            let controller: FlutterViewController = window?.rootViewController as! FlutterViewController
            let channel = FlutterMethodChannel(name: "your_wallet_channel", binaryMessenger: controller.binaryMessenger)
            channel.setMethodCallHandler { (call: FlutterMethodCall, result: @escaping FlutterResult) in
                if call.method == "showAddToWalletButton" {
                    WalletMethods.showAddToWalletButton()
                } else {
                    result(FlutterMethodNotImplemented)
                }
            }
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}


import PassKit

class WalletMethods {
    static func showAddToWalletButton() {
        let passButton = PKAddPassButton(addPassButtonStyle: .black)

        passButton.frame = CGRect(x: 20, y: 20, width: 200, height: 44)

        passButton.addTarget(self, action: #selector(addToWalletButtonTapped), for: .touchUpInside)

        let controller = UIApplication.shared.keyWindow?.rootViewController
        controller?.view.addSubview(passButton)
    }

    @objc private func addToWalletButtonTapped() {
        
    }
}

Also I have created channel for connection between native code.

class WalletChannel {
  static const MethodChannel _channel = MethodChannel('your_wallet_channel');

  static Future<void> showAddToWalletButton() async {
    await _channel.invokeMethod('showAddToWalletButton');
  }
}

And finally my screen is like this

class _WalletScreenState extends State<WalletScreen> {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      showAddToWalletButton();
    });
  }

  void showAddToWalletButton() {
    WalletChannel.showAddToWalletButton();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Wallet Screen')),
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text('Welcome to Your Wallet'),
          SizedBox(height: 20),
          Text('This is your digital wallet.'),
          SizedBox(height: 20),
          Text('Manage your cards and passes.'),
          SizedBox(height: 40),
        ],
      ),
    );
  }
}