Flutter Webview - How to postMessage from Dart to JS?

I'm trying to pass the FCM data from dart to the webview using postMessage and listen to these messages in ReactJS.

ReactJS:

window.addEventListener("message", () => {});

Flutter:

webViewController.runJavascript('window.postMessage("Hello ReactJS")');

Unfortunately, no events going through to ReactJS. Does someone have an idea how to use postMessage with webview_flutter?

UPDATED

        WebView(
          initialUrl: '<WEB_URL>',
          onPageStarted: (url) {
            setState(() {
              loadingPercentage = 0;
            });
          },
          onProgress: (progress) {
            setState(() {
              loadingPercentage = progress;
            });
          },
          onPageFinished: (url) {
            setState(() {
              loadingPercentage = 100;
            });
          },
          onWebViewCreated: (WebViewController controller) {
            webViewController = controller;

            Future.delayed(Duration(seconds: 5), () {
              print('JETZT');
              JavaScriptHelper.postMessage({'action': 'FCM_OPENED'});
            });
          },
          debuggingEnabled: true,
          javascriptMode: JavascriptMode.unrestricted,
        ),

JavaScriptHelper:

class JavaScriptHelper {
  static postMessage(Map<String, dynamic> data) async {
    try {
      print(json.encode(data));
      await webViewController?.runJavascript(
        'window.postMessage(${json.encode(data)})',
      );
    } catch (err) {
      print(err);
    }
  }
}

That was a thinking mistake on my side. The runJavascript method really executes the command in the Webview javascript therefore there is no window.postMessage because there is no iframe.

I managed it the following way:

Flutter:

controller.runJavascript('window.fromFlutter("Hello ReactJS")');

ReactJS:

(In App.tsx, but over render component)

(window as any).fromFlutter = function(data) {
 alert("This is working now!!!");
 alert(data);
}