It depends. You can just do a cross-domain form submission (set the action
to a page on another domain), or you can do server-server communication, or you can use JSONP (JSON wrapped in a function call).
VB.NET code for a Pastebin submission is:
Dim req As HttpWebRequest = DirectCast(WebRequest.Create("http://pastebin.com/api_public.php"), HttpWebRequest)
req.ContentType = "application/x-www-form-urlencoded"
req.Method = "POST"
Dim postData As String = "paste_code=Simple Example"
Dim postBytes As Byte() = Encoding.UTF8.GetBytes(postData)
req.ContentLength = postBytes.Length
Dim reqStream As Stream = req.GetRequestStream()
reqStream.Write(postBytes, 0, postBytes.Length)
reqStream.Close()
Dim resp As HttpWebResponse = DirectCast(req.GetResponse(), HttpWebResponse)
Dim respText As String = New StreamReader(resp.GetResponseStream(), Encoding.UTF8).ReadToEnd()
respText
is the generated paste bin URL. This can obviously be improved. It's an initial demonstration.