[FTP/FTPS Only] Completes the asynchronous operation started in BeginLogin.
Before calling BeginLogin, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginLogin returns. The callback method must accept the IAsyncResult returned by the BeginLogin method as a parameter. This parameter may then be passed to the EndLogin method to complete the operation.