Autofill username password web form

I'm developing program and I'm using Skybound Gecko browser ! i want to inter the username and password automatically when i open the page!

I'm using this code to do this :

Private Sub login_Click(sender As Object, e As EventArgs) Handles login.Click
GeckoWebBrowser1.Document.GetElementById("codeTerminal").SetAttribute("value", "xxxxxx")
GeckoWebBrowser1.Document.GetElementById("userName").SetAttribute("value", "xxxxxx")
GeckoWebBrowser1.Document.GetElementById("pass").SetAttribute("value", "xxxxxx")

End Sub i test this on google and on Facebook its work !! but not working with the site i want :(

this is the site : http://ift.tt/1K09tzM

when i use the above code i face this error :

An unhandled exception of type 'System.NullReferenceException' occurred in Noor Phone.exe

Additional information: Object reference not set to an instance of an object.

this the web page text information :

1-"<"input name="codeTerminal" type="text" id="codeTerminal" style="float:left;display:inline;background-image:url('imges/login_box_white.png');width:243px;height:44px;background-repeat:no-repeat;border:none;line-height:44px;">"

2-"<"input name="userName" type="text" id="userName" style="float:left;display:inline;background-image:url('imges/login_box_white.png');width:243px;height:44px;background-repeat:no-repeat;border:none;line-height:44px;">

3-"<"input name="pass" type="password" id="pass" style="float:left;display:inline;background-image:url('imges/login_box_white.png');width:243px;height:44px;background-repeat:no-repeat;border:none;line-height:44px;">

how i can fix this i want this so badly

any help please..