Dim forms As HtmlElementCollection = _
    wb.Document.All.GetElementsByName("name_der_form")
If forms.Count > 0 Then
    forms(0).InvokeMember("submit") 'Absenden
End If

Dim elementsLogin As HtmlElementCollection = _
    wb.Document.All.GetElementsByName("login_login") 'Username
elementsLogin(0).SetAttribute("value", tbUser.Text)
 
Dim elementsPassword As HtmlElementCollection = _
  wb.Document.All.GetElementsByName("login_password") 'Password
elementsPassword(0).SetAttribute("value", tbPassword.Text)
 
 
Dim parent As HtmlElement = elementsPassword(0) 'übergeordnetes <form> suchen
While Not String.Equals(parent.TagName, "form", _
  StringComparison.InvariantCultureIgnoreCase)
    parent = parent.Parent
End While
parent.InvokeMember("submit")'Formular absenden