Untrusted HTTPS certificate

Hi,

I use System.Net.HttpClient to call my webservice in my PCL Forms project.
With HTTP everything works great, but now for the stagging environment the WS was passed in HTTPS with an untrusted certificate (it will be a trusted one in production).

I haven't seen any way online or in the forum to call an untrusted webservice in Xamarin.Forms.
The only thing i have seen is these kind of fix :

System.Net.ServicePointManager.ServerCertificateValidationCallback =
                new System.Net.Security.RemoteCertificateValidationCallback(delegate { return true; });

But the ServicePointManager namespace is not available in PCL project.

Did anyone ever get that issue ? How did you solve it ?
Regards