Cannot find RemoteCertificateValidationCallback in ClientWebSocketOptions

        Socket = new ClientWebSocket();
        Socket.Options.RemoteCertificateValidationCallback = delegate { return true; };

'ClientWebSocketOptions' does not contain a definition for 'RemoteCertificateValidationCallback' and no accessible extension method 'RemoteCertificateValidationCallback' accepting a first argument of type 'ClientWebSocketOptions' could be found (are you missing a using directive or an assembly reference?)