Silverlight 5 OOB web browser Fill Form

Silverlight 5 OOB web browser Fill Form

I am using webbrowser control in Silverlight 5, in the out of browser app,
to navigate to specific website and fill form:
string UrlString = "<iframe src = \"https://somewebsie.com/login\" width =
\"100%\" height = \"100%\"> </iframe>";
HtmlContent.NavigateToString(UrlString2);
After the page is loaded I am trying to fill the input element:
private void bnt_Login_Click(object sender, RoutedEventArgs e)
{
HtmlDocument doc = HtmlPage.Document;
HtmlElement userName = doc.GetElementById("inputUnserName");
userName.SetAttribute("value", "me");
}
However, after pressing the button I am getting blank screen in SL5. How
can I fillout the form in SL5? Thank you.