[FTP/FTPS Only] Completes the asynchronous operation started in BeginSendUserName.
Before calling BeginSendUserName, 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 BeginSendUserName returns. The callback method must accept the IAsyncResult returned by the BeginSendUserName method as a parameter. This parameter may then be passed to the EndSendUserName method to complete the operation.