This is a simple code in VB.net that send Http POST to a webpage and display the response.

Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click
Dim myWebReq As HttpWebRequest
Dim myWebResp As HttpWebResponse
Dim encoding As New System.Text.ASCIIEncoding()
Dim postData As String
Dim data() As Byte
Dim sr As StreamReader
Dim sw As StreamWriter

postData = "username=b"
postData += "&"
postData += "password=k"
data = encoding.GetBytes(postData)

myWebReq = WebRequest.Create("http://localhost/authenticate")
myWebReq.Method = "POST"
myWebReq.ContentType = "application/x-www-form-urlencoded"
myWebReq.ContentLength = data.Length

Dim myStream As Stream = myWebReq.GetRequestStream()
myStream.Write(data, 0, data.Length)
myStream.Close()
myWebResp = myWebReq.GetResponse
sr = New StreamReader(myWebResp.GetResponseStream)
Dim strHTML As String = sr.ReadToEnd
MessageBox.Show(strHTML)
End Sub
This code is invoked when user submit a button, easy and simple