Setting Proxy username and password

[iotainsanity]

Here is the code I have.

Public Class Form1
 <DllImport("User32.dll")> _
    Public Shared Function FindWindow(ByVal lpClassName As String, ByVal lpWindowName As String) As Int32
    End Function
#Region "Using Proxy"
    <Runtime.InteropServices.DllImport("wininet.dll", SetLastError:=True)> _
    Private Shared Function InternetSetOption(ByVal hInternet As IntPtr, ByVal dwOption As Integer, ByVal lpBuffer As IntPtr, ByVal lpdwBufferLength As Integer) As Boolean
    End Function
    Public Structure Struct_INTERNET_PROXY_INFO
        Public dwAccessType As Integer
        Public proxy As IntPtr
        Public proxyBypass As IntPtr
    End Structure

    Private Sub UseProxy(ByVal strProxy As String)
        Const INTERNET_OPTION_PROXY As Integer = 38
        Const INTERNET_OPEN_TYPE_PROXY As Integer = 3
        Dim struct_IPI As Struct_INTERNET_PROXY_INFO
        Dim credentialstringvalue As String = (proxyusername & ":" & proxypassword)
        Dim credentialbytearray = ASCIIEncoding.ASCII.GetBytes(credentialstringvalue)
        Dim credentialbase64string = Convert.ToBase64String(credentialbytearray)
        Dim nullObject As Object = 0
        Dim nullObjectString As Object = ""
        Dim authObject As Object = String.Format("Proxy-Authorization: Basic {0}{1}", credentialbase64string, Environment.NewLine)
        struct_IPI.dwAccessType = INTERNET_OPEN_TYPE_PROXY
        struct_IPI.proxy = Marshal.StringToHGlobalAnsi(strProxy)
        struct_IPI.proxyBypass = Marshal.StringToHGlobalAnsi("local")
        Dim intptrStruct As IntPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf(struct_IPI))

        Marshal.StructureToPtr(struct_IPI, intptrStruct, True)

        Dim iReturn As Boolean = InternetSetOption(IntPtr.Zero, INTERNET_OPTION_PROXY, intptrStruct, System.Runtime.InteropServices.Marshal.SizeOf(struct_IPI))
    End Sub
#End Region

<<<This works perfectly but im using a private proxy, so when my webbrowser naviagtes to a website the Windows security box pops up asking for username password.>>>

How can I automatically input the proxy username(proxyusername) and proxypassword(proxypassword)

I tried doing the following

Dim mycreds As New CredentialCache
        Dim proxyauth As New NetworkCredential("user", "pass")
        CredentialCache.DefaultNetworkCredentials.UserName = "user"
        CredentialCache.DefaultNetworkCredentials.Password = "pass"
        CredentialCache.DefaultNetworkCredentials.Domain = "proxyip"
        UseProxy("proxy,port")
        WebBrowser1.Navigate("http://www.whatismyip.com")