xamarin.ios httpclient clientcertificate not working with https

Question

I have a webservice which only allows https and ssl requests. 1&1 generated us an certificate from Geo Trust. (.pfx)

In Android my https request are working with the certificate. It Looks like this:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Ssl3;

        using (var mmstream = new MemoryStream())
        {
            Android.App.Application.Context.Assets.Open("***.pfx").CopyTo(mmstream);
            byte[] b = mmstream.ToArray();

            X509Certificate2 cert = new X509Certificate2(b, "'''", X509KeyStorageFlags.DefaultKeySet);
            clientHandler.ClientCertificates.Add(cert);
        }


        ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback((sender, certificate, chain, policyErrors) => { return true; });

But in iOS it doesnt work. Heres the Code

HttpClientHandler clientHandler = new HttpClientHandler() { ClientCertificateOptions = ClientCertificateOption.Automatic };
        using (var mmstream = new MemoryStream())
        {
            Assembly ass = Assembly.GetExecutingAssembly();
            string[] str = ass.GetManifestResourceNames();
            foreach (string resource in str)
            {
                if (resource.Contains("SSL"))
                    ass.GetManifestResourceStream(resource).CopyTo(mmstream);
            }

            byte[] b = mmstream.ToArray();

            X509Certificate2 cert = new X509Certificate2(b, "***", X509KeyStorageFlags.DefaultKeySet);
            clientHandler.ClientCertificates.Add(cert);

            return clientHandler;
        }

I always get the error Message The Methode is not implemented. (line clienthandler.clientcertificated.add(cert);).

What am i doing wrong?