Click Object on Web Page in Xamarin forms

Question

I wan to click button which created in web page. I added following code to the my project but it did execute, string always null.

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
        wbWeb.Source = "https://www.facebook.com";
    }

    private async void btnBrowser_Clicked(object sender, EventArgs e)
    {             
       string str = wbWeb.EvaluateJavaScriptAsync($"document.getElementById('loginbutton').click();");
      //str is null
    }
}