Communicate between Wrapped angular web app to flutter app (webview_flutter)

Question

• I have wrapped the angular web app into flutter app using webview_flutter and WebViewWidget
• Now I need to send the string message angular app to flutter - while click the button in web app
• I have tried but not working and attached the tried snippet

In flutter :

class _MyHomePageState extends State<MyHomePage> {
  WebViewController controller = WebViewController()
        ..setJavaScriptMode(JavaScriptMode.unrestricted)
        ..setBackgroundColor(Color.fromRGBO(0, 0, 0, 0))
        ..setNavigationDelegate(
          NavigationDelegate(
            onProgress: (int progress) {
              print("--------------onProgress-----------------");
            },
            onPageStarted: (String url) {
              print("--------------onPageStarted-----------------");
            },
            onPageFinished: (String url) {
              print("--------------onPageFinished-----------------");
            },
            onWebResourceError: (WebResourceError error) {
              print(-------------onWebResourceError-------------);
            },
            onNavigationRequest: (NavigationRequest request) {
              return NavigationDecision.navigate;
            },
          ),
        )
        ..loadRequest(Uri.parse('my url'))
        ..addJavaScriptChannel(
          "imageDownload",
          onMessageReceived: (JavaScriptMessage message) {
            print("-------------Channel-------------------");
            print(message.message);
          },
        );
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: WebViewWidget(controller: controller),
    );
  }
}

In angular :

if(userAgent.includes('Win')){
////some windows related code
}else if (userAgent.includes('Android')) {
   window.parent.postMessage("Check message is receiving", '*');
}