SessionOptions sessionOptions = new SessionOptions
{
  Protocol = Protocol.Sftp,
  HostName = sftpHostName, //Destination server to which we need to connect to
  UserName = sftpUserId, //Destination server UserId (or Source server user id?)
  SshHostKeyFingerprint = sftpHosyKeyFingerPrint,
  SshPrivateKeyPath = sftpPrivateKeyPath //Entire physical path that maps to .ppk private key (Eg: C:\PrivateKey\SFTPKey.ppk)
};
 
//Start Session
using (Session session = new Session())
{
  session.ExecutablePath = executablePath;
  session.Open(sessionOptions);
 
  // Upload files
  TransferOptions transferOptions = new TransferOptions();
  transferOptions.TransferMode = TransferMode.Binary;
  TransferOperationResult transferResult;
  transferResult = session.PutFiles(fileToUpload, sftpDestinationServerPath, false, transferOptions);
 
  // Throw on any error
  transferResult.Check();
}