string host = "seu_host";
string username = "seu_usuario";
string password = "sua_senha";
MemoryStream fileData = new MemoryStream();
string remoteFilePath = "/caminho/remoto/no/servidor/nome_do_arquivo.txt";
var connectionInfo = new ConnectionInfo(host, port, username, new PasswordAuthenticationMethod(username, password));
using (var client = new SftpClient(connectionInfo))
client.UploadFile(fileData, remoteFilePath);
Console.WriteLine("Arquivo enviado com sucesso via SFTP.");