webbrowser.Navigate( urlString As String,targetFrameName as String, postData As _
  Byte(), additionalHeaders As String )

Dim url as String = http://www.irgendwas.de/index.php
Dim Header As String = "Content-Type: application/x-www-form-urlencoded"
Dim frame As String = "_self"
Dim text as String = "text&=%texttext"
Dim post as String = "sendtext=" & text
Dim postByte As Byte()
postByte = ASCIIEncoding.ASCII.GetBytes(post)
wenbrowser.Navigate(url, frame, postByte, Header)