To create a *.torrent file on a seedbox with rTorrent you need to use the "Create" icon in the bottom menu. Just click on it and then such a menu should open:
Now you need to point to the path to files from which you want to make *.torrent file in "Select Source" field, click on next to "Select Source" field and then a menu will drop down, choose the file/catalogue you want to place in the torrent like on the screenshot:
Now type the tracker address and tick "Private torrent" if you wish to seed on a private tracker. Then click on "Create".
This will bring up the create console menu which shows the progress and status of the process, and displays any problems if they occur.
When this is complete click on "Save" to download created *.torrent file which you can use to upload to whatever tracker you created the torrent for.