Assuming you have already integrated the Scrimmage Rewards Widget into your Flutter application, you can listen to events emitted by the widget by adding a JavaScript interface to the WebView.

var message = '';

// ...

  // ...
  javascriptChannels: Set.from([
      // Use ScrimmageChannel for communication
      name: 'ScrimmageChannel',
      onMessageReceived: (JavascriptMessage message) {
        setState(() {
          this.message = message.message;
          print('Received message from WebView: ${message.message}');

Check out the Widget Events API for more information.