Hi,
Just to clarify. You are asking how to first upload a torrent and then seed it from your seedbox?
To upload a torrent there are already
several guides in the forum that should be helpful.
Once you have uploaded the torrent, you download the .torrent file on the tracker from your uploaded torrent and add it to your seedbox. If the files are already on the seedbox it will recheck and then seed it. So you must first transfer the files to the seedbox before adding the .torrent file. Ask your seedbox support if you have doubts on how to do this!