Click the torrent link to open which should start the Launch Application window where you should check mark the Remember my choice for magnetic links, and click the Choose... button;

Now in the next window that appears you can browse to and select the file /usr/bin/qbittorrent. For this click File System on the side pane and browse through the directories usr -> bin where you'll have to select the executable file qbittorrent to open.

Or, just click the File System and click the "edit path" icon to type a file name manually (or use the shortcut Ctrl + L);

Then in the path input box labelled Location: type the path of the executable of the application - /usr/bin/qbittorrent and click Open;

Having the desired launch application selected, now click the OK button. This will start the application qBittorrent and you can continue managing your download from there.