Add Proxy to HttpWebResponse - vb.net

Question

When using a WebRequest I can add the system default proxy and the default credentials like this
WebRequest = System.Net.WebRequest.Create(URL)
WebRequest.Method = "GET"
WebRequest.ContentType = "application/x-www-form-urlencoded"
WebRequest.Headers.Add("...some headers here...")
WebRequest.Credentials = CRED
WebRequest.Proxy = System.Net.WebRequest.GetSystemWebProxy
WebRequest.Proxy.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials
Public Shared Function MultipartFormDataPost(ByVal postUrl As String, ByVal postParameters As Dictionary(Of String, Object)) As HttpWebResponse
Dim formDataBoundary As String = String.Format("----------{0:N}", Guid.NewGuid())
Dim contentType As String = "multipart/form-data; boundary=" & formDataBoundary
Dim formData As Byte() = GetMultipartFormData(postParameters, formDataBoundary)
Return PostForm(postUrl, contentType, formData)
End Function
Private Shared Function PostForm(ByVal postUrl As String, ByVal contentType As String, ByVal formData As Byte()) As HttpWebResponse
Dim request As HttpWebRequest = TryCast(WebRequest.Create(postUrl), HttpWebRequest)
If request Is Nothing Then
Throw New NullReferenceException("request is not a http request")
End If
request.Method = "POST"
request.ContentType = contentType
request.Credentials = CRED
request.Headers.Add(EBC_TP_ZUGANG)
request.CookieContainer = New CookieContainer()
request.ContentLength = formData.Length
request.Proxy = System.Net.WebRequest.GetSystemWebProxy
request.Proxy.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials
Using requestStream As Stream = request.GetRequestStream()
requestStream.Write(formData, 0, formData.Length)
requestStream.Close()
End Using
Return TryCast(request.GetResponse(), HttpWebResponse)
End Function
What do I have to do when I need to add that proxy to a HttpWebResponse?
This is my code:
Dim postURL As String = "https://www.server.com/blabbla"
Dim webResponse As HttpWebResponse = clsFileUpload.MultipartFormDataPost(postURL, postParameters)
Dim responseReader As StreamReader = New StreamReader(webResponse.GetResponseStream())
Dim fullResponse As String = responseReader.ReadToEnd()
webResponse.Close()