VB.net FCM proxy 407 issue

Question

I have a basic setup for Push Delivery, they are working fine without proxy. I tried to add a proxy setting but couldn't manage to make it work.

I am using below function to deliver push notifications:

Public Function Send(ByVal title As String, body As String, token As String) As String
    Dim retval As String
    Try
        If FirebaseApp.DefaultInstance Is Nothing Then
            Dim defaultApp = FirebaseApp.Create(New AppOptions() With {
    .Credential = GoogleCredential.FromFile(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "key.json")), .HttpClientFactory = New ProxySupportedHttpClientFactory()
    })
        End If
        Dim message As New Message
        With message
            .Webpush = New WebpushConfig
            With message.Webpush
                .Notification = New WebpushNotification
                With message.Webpush.Notification
                    .Title = title
                    .Body = body
                End With
            End With
        End With
        Dim messaging = FirebaseMessaging.DefaultInstance
        message.Token = token
        Dim result = messaging.SendAsync(message)
        retval = result.Result
    Catch ex As Exception
        retval = "ERROR: " & ex.Message
        If Not ex.InnerException Is Nothing Then
            If ex.InnerException.Message = "Requested entity was not found." Then
                retval = "ERROR: Requested token is not registered or invalid."
            Else
                retval &= "INNER Exception: " & ex.InnerException.Message
            End If
        End If
    End Try
    Return retval
End Function

And using below class for proxy integration:

Public Class ProxySupportedHttpClientFactory
Inherits HttpClientFactory


Protected Overrides Function CreateHandler(ByVal args As CreateHttpClientArgs) As HttpMessageHandler

    credentials = New NetworkCredential("1", "1")
    Dim proxy = New WebProxy("http://127.0.0.1:8888", True, Nothing, credentials)
    Dim webRequestHandler = New HttpClientHandler() With {
        .UseProxy = True,
        .Proxy = proxy,
        .UseCookies = False
    }
    Return webRequestHandler
End Function
End Class

But when I try to send a notification I get "The remote server returned an error: (407) Proxy Authentication Required." error in line "retval = result.Result".

Proxy configuration is working fine with WebProxy with a simple request test but not with HttpClientHandler in FCM.

EDIT: I am using authorization with key.json file in credentials, articles says i should implement proxy for credential connectivity too but i couldn't find any way to integrate it with credential part.