_process.StartInfo.UserName = "secureUserName";
_process.StartInfo.Password = new System.Security.SecureString();
 
string passwd = "securePassword";
foreach (char c in passwd)
{
     _process.StartInfo.Password.AppendChar(c);
}
_process.StartInfo.Domain = "OurDomain";

_process.StartInfo.UserName = ...;
_process.StartInfo.Password = ...;

Impersonator impersonator = new Impersonator("secureUser", "ourDomain", "secureUserPassword"); 
 
//
// Other code here runs correctly as 'secureUser'
//
 
// Then this is running WinSCP as the logged in user. We know this because
// We're getting access denied for the folder on our network that we're trying to download to.
// I can also see WinSCP in task manager under the logged in user
// If an admin user logs in then the FTP download works.
 
using (WinSCP.Session session = new WinSCP.Session())
{
    // Connect               
    session.Open(sessionOptionsForExternalFTPSite);
 
    RemoteDirectoryInfo rd = session.ListDirectory(sessionOptionsForExternalFTPSite);
    RemoteFileInfoCollection rfc = rd.Files;
 
    foreach (RemoteFileInfo rf in rfc)
    {
        if (rf.Name.Contains(fromFileNameStart))
        {
              fileList.Add(rf.Name);
        }
    }
 
    TransferOptions transferOptions = new TransferOptions();
    transferOptions.TransferMode = TransferMode.Binary;
    transferOptions.FileMask = fileMask; 
 
    transferResult = session.GetFiles(FTPDownloadDir, toFilePath, false, transferOptions);
 
    // Throw on any error
    transferResult.Check(); // Access denied error!
}
 
return fileList; //