Ok, after a bit of experimenting this is what I would recommend, with Transmission :
Right-click on the torrent, open properties.
Go to the tracker tab.
If empty, click add and past
(Careful not to add spaces before or after, Transmission doesn't seem to like it.)
Validate, close, right click again on the torrent, and copy the magnet link.
Past it here, this time it should mention the tracker somewhere at the end, which was not the case with the previous one.
Hope that works !