Bla bla bla, boring and very long introduction that no ones cares about.
Install openfortivpn from Debian's repositories:
sudo apt install openfortivpn
Open a web browser and log in to your organization's fortinet SSL-VPN portal. Right click anywhere on the page, Inspect, and go to the Network tab in order to copy the SVPNCOOKIE= cookie.
Now open a terminal and run:
sudo openfortivpn --cookie SVPNCOOKIE=....
Sometimes the connection may fail with the following error:
ERROR: Could not get VPN configuration (HTTP status code).
For some reason this often happens to me when I use Firefox to grab the cookie, however I have no problem with google-chrome. I haven't tested other browsers.