check
An FTP secure client library for C#[
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if you not using any other library. when you create request set below properties and credentials
FtpWebRequest request = (FtpWebRequest)WebRequest.Create(serverUri);
request.EnableSsl = true;
request.UsePassive = true;
request.UseBinary = true;
request.KeepAlive = true;
request.Credentials = new NetworkCredential(_userName, _password);