HttpClient send request throws Exception from HRESULT: 0x80072F8F

I'm trying to send a web request to an ActiveSync server using HttpClient in Windows Universal App (Windows 8.1/Windows Phone 8.1). When i call the method SendRequestAsync it throws Exception

"HResult:0x80072F8F"

and the response is null.

I found that in the RequestMessage the HttpRequestMessage.TransportInformation property contains some information. HttpRequestMessage.TransportInformation.ServerCertificate contains a value and HttpRequestMessage.TransportInformation.ServerCertificateErrors contains an Entry with ChainValidationResult.Untrusted.

The situation I am trying to solve is: I do not have any certificates. I call an unknown server with HttpClient and that server responds "ChainValidationResult.Untrusted". How can i use the HttpRequestMessage.TransportInformation.ServerCertificate in order to make the call successful?