using SHDocVw; // Don't forget

InternetExplorer IEControl = new InternetExplorer();
IWebBrowserApp IE = (IWebBrowserApp)IEControl;
IE.Visible = true;

// Convert the string into a byte array
ASCIIEncoding Encode = new ASCIIEncoding();
byte[] post = Encode.GetBytes("username=fabio&password=123");

// The destination url
string url = "http://example.com/check.php";

// The same Header that its sent when you submit a form.
string PostHeaders = "Content-Type: application/x-www-form-urlencoded";

IE.Navigate(url, null, null, post, PostHeaders);

<?php
echo $_POST['username'];
echo " ";
echo $_POST['password'];
?>

<%
response.write(Request.Form("username"))
response.write(" " & Request.Form("password"))
%>