Hola.
Hace tiempo publiqué en el foro este ejemplo de uso de GeckoFx para rellenar los controles del form de login de elhacker.net, espero que te sirva de algo:
' Navigate to the Formulary.
GeckoWebBrowser1.Navigate("http://foro.elhacker.net/login.html")
' ( Wait until the page is fully loaded before continue )
' Get the username textbox and set the attribute value.
GeckoWebBrowser1.Document.GetElementsByName("user").First.SetAttribute("value", "My Username")
' Get the password textbox and set the attribute value.
GeckoWebBrowser1.Document.GetElementsByName("passwrd").First.SetAttribute("value", "My Password")
' Click the last button to submit.
'GeckoWebBrowser1.Document.GetElementsByTagName("Input").Last.Click()
' Or submit the Formulary this way.
GeckoWebBrowser1.Document.GetElementsByTagName("Form").Cast(Of GeckoFormElement).First.submit()
Saludos!