Last I looked at this there were issues around setting the private flag (which I'm not opposed to removing as it's mostly there for historical reasons). The closest I could reliably do at the time was using the 'xs' parameter to link to an actual .torrent file.
This might not be exactly what you're looking for, but it can be enabled from the settings page:
Your posts also drew attention to a parsing issue with magnet links in comments. That will be fixed soon (edit: done).