C# WebProxy not works with https

Question

I have Client + Win service.
Both must work with web server using proxy.
Without proxy all works fine,
With system proxy settings (proxy settings from ie)

WebRequest.DefaultWebProxy

Client works fine, but service don't see this proxy settings (netsh winhttp set proxy don't help me). So - proxy server works OK
When I trying to use manual settings:

var_proxy = new WebProxy(Server + ":" + Port, true)
            {Credentials = null};
var request = (HttpWebRequest)WebRequest.Create(target);
        request.ContentType = Constants.ContentType;   // Default content type
        request.UserAgent = _userAgentHeader;
        request.Method = "POST";
        request.Proxy = _proxy;

And as I see in proxy logs - It works with http server, but not used with https! All requests go directly. How can I fix it?