Untitled

WebBrowser1.DocumentText =
    "<html><body>Search in Google:<br/>" &
    "<form method='get' action='http://www.google.com/search'>" &
    "<input type='text' name='as_q'/><br/>" &
    "<input type='submit' value='Search'/>" &
    "</form></body></html>"

End Sub

Dim document = WebBrowser1.Document
    If document IsNot Nothing And
    document.All("as_q") IsNot Nothing And
    String.IsNullOrEmpty(
    document.All("as_q").GetAttribute("value")) Then
        e.Cancel = True
        MsgBox("Please enter a search term.")
    End If

End Sub