@OsamaMunawar: As the tracker link above shows, there's now Session.PutFile for uploading from a stream. Though so far it works with SFTP only.
https://winscp.net/eng/docs/library_session_putfile

Request to support FTP too is tracked. You can vote for it here:
https://winscp.net/tracker/1945

I was working on your library and found you have done great contribution. Thank you for that. Can you please tell when this can be achieved, if we want to upload a folder or zip file in stream to FTP or SFTP using PutFiles function?

This request is being tracked here:
https://winscp.net/tracker/1738
You can vote for it there.

Still not possible?

No, it's not possible.

Is there a way to upload files using a memory stream and filename using the WinSCP library?

The reason is to avoid writing to disk so the process will be seamless through the file transfer. If there is a function that would be a good help.

Thanks!