Post com WebClient

post.cs

using (var client = new WebClient())
{
	client.Credentials = new NetworkCredential("SeuUsuario", "SuaSenha");

	var url = "UrlServico";
	var parameters = "op=Consulta&agencia=J3&origem=1&destino=57&data=2013-02-15";

	var response = client.UploadString(
			url,
			"POST",
			parameters);
}