HTML Injection Windows RT WebView

I am trying to insert values to a HTML Document on my Windows Universal App, the HTML Document is located on a Web-server.

And i am trying to insert username and password saved on the client device Winphone or PC Store App, I know the id and every thing about the elements, but I cant figure out how to insert a string into an input value, can someone provided a sample with InvokeScript("smth", new string[] {smthelse});

Or has someone a better idea to insert username and password from into the HTML content of a webview?

I'am stuck here and i really need this feature.

Thanks in advance

via Chebli Mohamed