when I look at your code, I see the last in BindingElementCollection is
TextMessageEncodingBindingElement()
try it on the following way:
1) remove the new HttpsTransportBindingElement() from the construtor
2) add this element as the last element after
Cusbinding.Elements.Add(new TextMessageEncodingBindingElement()
{
MessageVersion = System.ServiceModel.Channels.MessageVersion.Default,
WriteEncoding = System.Text.Encoding.UTF8
});
Cusbinding.Elements.Add(new HttpsTransportBindingElement());