I would like my custom browser to auto fill in a form when it is completely loaded

Ok so inside

   private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    { }

Ive inserted the following statements

webBrowser1.Document.GetElementById("FirstName").SetAttribute("value", "John");
webBrowser1.Document.GetElementById("LastName").SetAttribute("value", "Smith");
// etc..etc..

I noticed that "webBrowser1_DocumentCompleted" only is loaded one time?? How do i make my browser auto fill in a form when the document has finish loading, and auto fill the values to the define values if they have been changed by the end user.