Cross domain issue?

code behind...
Imports Microsoft.VisualBasic
Imports System.Net
Imports System.Net.Security
Imports System.Security.Cryptography.X509Certificates
Imports System.Web.Security
 
Partial Class _Default
    Inherits System.Web.UI.Page
 
    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim mypost As New XMLTest
        Dim RESPONSE_TEXT As String = "<xml><set><callrequest_update><request>Call-Say-Time</request><request_id>xxxxxxx</request_id><orig_address>xxxxx</orig_address><request_address>xxxxxx</request_address><request_status>pending</request_status></callrequest_update></set><src></src></xml>"
        TextBox1.Text = mypost.sendPost(RESPONSE_TEXT)
    End Sub
End Class
Public Class XMLTest
    Public Const URL_BASE = "https://sanvtest.vincomm.net/tac/SiPbxDomain?kind=subscriber_config"
    Public Const RSP_USERNAME As String = "xxxxx"
    Public Const RSP_PASSWORD As String = "xxxxx"
    Public Const PRIVATE_KEY = "Enter Private Key"
    Public Function sendPost(ByVal str As String)
        Dim myHttpWebRequest As New System.Net.WebClient
        Dim rs As String = ""
        Dim myCache As New CredentialCache
        myCache.Add(New Uri(URL_BASE), "Basic", New NetworkCredential(RSP_USERNAME, RSP_PASSWORD))
 
        myHttpWebRequest.Credentials = myCache
 
        System.Net.ServicePointManager.ServerCertificateValidationCallback = New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate)
        myHttpWebRequest.Headers.Add("Content-Type", "text/xml")
        Try
            Dim sendData As Byte() = System.Text.Encoding.ASCII.GetBytes(str)
            Dim myHttpResponse As Byte() = myHttpWebRequest.UploadData(URL_BASE, "POST", sendData)
            rs = System.Text.Encoding.ASCII.GetString(myHttpResponse)
        Catch ex As Exception
            rs = "err:" + rs
        End Try
        Return rs
    End Function
    'Used to convert to MD5
    Public Function cMD5(ByVal str As String) As String
        'Must have Imports System.Web.Security in General Declarations
        Dim Hash As String = FormsAuthentication.HashPasswordForStoringInConfigFile(str, "MD5")
        Return Hash.ToLower
    End Function
    Public Shared Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
        Return True
    End Function
End Class