The existing answers are valid, but why re-invent the wheel and bother with lower level WebRequest
types while WebClient
already implements FTP uploading neatly:
using (var client = new WebClient())
{
client.Credentials = new NetworkCredential(ftpUsername, ftpPassword);
client.UploadFile("ftp://host/path.zip", WebRequestMethods.Ftp.UploadFile, localFile);
}