'Variabili per la richiesta e la risposta FTP
Dim Request As Net.WebRequest
Dim Response As Net.WebResponse
Dim Credentials as New Net.NetworkCredentials("username","password")
Dim Uri_Del_File as New Uri("ftp://www.sito.com/file.txt")
'Creo la richiesta e passo le credenziali
Request = Net.WebRequest.Create(New Uri(Uri_Del_File))
Request.Credentials = Credentials
'Metodo AppendFile
Request.Method = Net.WebRequestMethods.Ftp.AppendFile
'Ottengo lo stream del file remoto su cui scrivere
Dim RemoteStream As IO.Stream = Request.GetRequestStream
'Scrivo sullo stream, puoi anche cambiare la codifica a seconda di cosa ti serve
Dim WriteStream As New IO.StreamWriter(RemoteStream, System.Text.ASCIIEncoding.ASCII)
WriteStream.Write(DataToAppend)
WriteStream.Close()