var client = new RestClient("url");
var request = new RestRequest(Method.PUT);
// Create SSL certificate 
// Certificate CN name is used to authenticate with Secure Gateway
string certificateFile = "<SSL_Certificate>.pfx";        
var certificate = new X509Certificate(certificateFile);
client.ClientCertificates = new X509CertificateCollection();
client.ClientCertificates.Add(certificate);