如何从xamarin android web视图中加载的登录页面的username文本框中读取值

using Android.App;
using Android.OS;
using Android.Webkit;
using Android.Widget;
using System.Threading.Tasks;

namespace XamarinWebViewExample
{
    [Activity(Label = "WebViewExample", MainLauncher = true)]
    public class MainActivity : Activity
    {
        private WebView webView;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.Main);

            webView = FindViewById<WebView>(Resource.Id.webview);
            webView.Settings.JavaScriptEnabled = true;
            webView.LoadUrl("https://example.com/login");

            webView.SetWebViewClient(new WebViewClient()
            {
                OnPageFinished = async (sender, e) =>
                {
                    await ReadUsername();
                }
            });
        }

        private async Task ReadUsername()
        {
            var js = @"
                function readUsername() {
                    var username = document.getElementById('username').value;
                    return username;
                }
            ";

            var username = await webView.EvaluateJavascriptAsync(js + "readUsername();");
            Toast.MakeText(this, "Username: " + username, ToastLength.Long).Show();
        }
    }
}