Since Ubuntu 18.04 (bionic), you can connect with the regular network manager by simply installing the package
network-manager-fortisslvpn-gnome. Find it in Software or do
sudo apt install network-manager-fortisslvpn-gnome
(This will also install dependencies openfortivpn and network-manager-fortisslvpn.)
Then you can click
Edit connections in network manager (right click the wifi icon), + to add a new one, select type
Fortinet SSLVPN under the
VPN heading. Give it a name, then under
Gateway put in the IP address (and optionally port separated by colon, e.g.
220.127.116.11:44443) and username/password. Note that you may have to click the little symbol in the password field to select who has access to the password before you're allowed to type anything in that text field.
Save and right-click the wifi icon again, click VPN and pick the name you chose.
If it doesn't work, that may be because you need to trust a certificate. If you have the certificate fingerprint, you can edit your connection and add it under
Advanced→Trusted certificate. If you don't know your certificate, https://serverfault.com/a/922767/104546 explains how to find it from the terminal.