set WATCH_BASE to the absolute path to your watch folder in torrentdrop.py.

run it on the remote server (running rtorrent) with something like:

$ gunicorn -w 1 -b 0.0.0.0:5050 torrentdrop:app

my server runs arch, so I made a rtdrop.service file in /etc/systemd/system/:

[Unit]
Description = Drop torrents files into rtorrent watch folders remotely by url

[Service]
WorkingDirectory = /home/ship/code/torrent-drop
ExecStart = /usr/bin/gunicorn torrentdrop:app -w 1 -b 0.0.0.0:5050

[Install]
WantedBy=multi-user.target

The main folder of this directory should also work as a chrome extension. Configure it on its options page, and then you should be able to right-click -> send to rtorrent on .torrent files!