Really I don’t use it, but one thing that comes to mind is to create a shared folder to be watched by transmission, so when you download some .torrent file through the browser and save it to this network share, transmission will add it to its queue.
This was tested on NethServer 6.8
Stop transmission
service transmission-daemon stop
Edit transmission settings (use your preferred text editor, here it’s nano)
nano -B /var/lib/transmission/settings.json
On the settings.json file append a comma after the current last option. Then, before the closing curly bracket, add:
"watch-dir": "/var/lib/nethserver/ibay/torrentwatch/",
"watch-dir-enabled": true
where torrentwatch is the name of the share.
Apply the changes:
signal-event nethserver-transmission-save