public static XmlElement GetToken(String[] args)
{
  // 1. Create an SSO server client-side object
  service = new STSService();
        
  // 2. Set the SSO server URL
  service.Url = args[0];
        
  // 3. SOAP Request Context - Required to add secruity headers
  SoapContext requestContext = service.RequestSoapContext;
        
  // 4. Create a CustomSecurityAssertion object that specifies username and password
  CustomSecurityAssertion objCustomSecurityAssertion = new CustomSecurityAssertion();
  objCustomSecurityAssertion.Username = args[1].Trim();
  objCustomSecurityAssertion.Password = args[2].Trim();
        
  // Use the assertions to set the policy
  Policy policy = new Policy();
  policy.Assertions.Add(objCustomSecurityAssertion);
  service.SetPolicy(policy);
        
  // 5. Establish a validation callback for the token certificate
  ServicePointManager.ServerCertificateValidationCallback +=
     new RemoteCertificateValidationCallback(ValidateRemoteCertificate);
        
        
  // 6. Create a token request
  RequestSecurityTokenType tokenType = new RequestSecurityTokenType();
        
  // Specify the token type, request type, key type, and signature algorithm
  tokenType.TokenType = TokenTypeEnum.urnoasisnamestcSAML20assertion;
  tokenType.RequestType = RequestTypeEnum.httpdocsoasisopenorgwssxwstrust200512Issue;
  tokenType.KeyType = KeyTypeEnum.httpdocsoasisopenorgwssxwstrust200512PublicKey;
  tokenType.SignatureAlgorithm = SignatureAlgorithmEnum.httpwwww3org200104xmldsigmorersasha256;
        
  // Set the token creation date/time
  LifetimeType lifetime = new LifetimeType();
  AttributedDateTime created = new AttributedDateTime();
  String createdDate =
     XmlConvert.ToString(System.DateTime.Now, XmlDateTimeSerializationMode.Utc);
  created.Value = createdDate;
  lifetime.Created = created;
        
  // Set the token expiration time
  AttributedDateTime expires = new AttributedDateTime();
  TimeSpan duration = new TimeSpan(1, 10, 10);
  String expireDate =
     XmlConvert.ToString(DateTime.Now.Add(duration), XmlDateTimeSerializationMode.Utc);
  expires.Value = expireDate;
  lifetime.Expires = expires;
        
  tokenType.Lifetime = lifetime;
        
  RenewingType renewing = new RenewingType();
  renewing.Allow = true;
  renewing.OK = true;
  tokenType.Renewing = renewing;
        
  // 7. Call Issue
  try
  {
     RequestSecurityTokenResponseCollectionType responseToken =
        service.Issue(tokenType);
     RequestSecurityTokenResponseType rstResponse =
        responseToken.RequestSecurityTokenResponse;
        
     return rstResponse.RequestedSecurityToken;
  }
  catch (Exception ex)
  {
     Console.WriteLine(ex.ToString());
     throw ex;
  }
}

CustomSecurityClientOutputFilter
constructor – Creates a token for the username and password. It also calls the
GetSecurityToken
method and creates a message signature for the security token.
SecureMessage
– An override method for the .NET method
SendSecurityFilter.SecureMessage
. The override method adds tokens and the message signature to the .NET Security element.
GetSecurityToken
– creates an X509 security token from a PFX certificate file. PFX is a Public-Key Cryptography Standard format that is used to store a private key and the corresponding X509 certificate.

internal class CustomSecurityClientOutputFilter : SendSecurityFilter
{
   UsernameToken userToken = null;
   X509SecurityToken signatureToken = null;
   MessageSignature sig = null;
        
   public CustomSecurityClientOutputFilter(CustomSecurityAssertion parentAssertion)
      : base(parentAssertion.ServiceActor, true)
   {
      userToken = new UsernameToken(parentAssertion.Username.Trim(),
                                    parentAssertion.Password.Trim(),
                                    PasswordOption.SendPlainText);
      signatureToken = GetSecurityToken();
      sig = new MessageSignature(signatureToken);
   }
        
   ///  SecureMessage
   public override void SecureMessage(SoapEnvelope envelope, Security security)
   {
      security.Tokens.Add(userToken);
      security.Tokens.Add(signatureToken);
      security.Elements.Add(sig);
   }
        
   /// GetSecurityToken - creates the security token from certificate from pfx file
   internal static X509SecurityToken GetSecurityToken()
   {
      X509Certificate2 certificateToBeAdded = new X509Certificate2();
      string certificateFile = ConfigurationManager.AppSettings["PfxCertificateFile"];
      certificateToBeAdded.Import(certificateFile, "", X509KeyStorageFlags.MachineKeySet);
      return new X509SecurityToken(certificateToBeAdded);
   }
}