I think the code below will solve my problem

TheModel model = new TheModel();
MemoryStream memoryStream = new MemoryStream();
XmlSerializer serializer = new XmlSerializer(typeof(TheModel));
serializer.Serialize(memoryStream, model);
FtpClient client = new FtpClient();
client.AutoConnect();
client.UploadStream(memoryStream, "", FtpRemoteExists.NoCheck, true);