How to connect SSL VPN on Ubuntu 18.04

As Fortinet VPN could use several VPN protocols
And as we do not know which one
-> You'd better use the Forticlient Official client for Linux & Ubuntu
Then you could export the vpn client xml configuration file from Windows that you will import to the Ubuntu client.

Find installation instructions for 18.04 here :
https://forticlient.com/repoinfo

Install Forticlient on Ubuntu 18.04 LTS

Install gpg key

wget -O - https://repo.fortinet.com/repo/ubuntu/DEB-GPG-KEY | sudo apt-key add - 

Add the following line in /etc/apt/sources.list

deb [arch=amd64] https://repo.fortinet.com/repo/ubuntu/ /bionic multiverse 

Update package lists

sudo apt-get update 

Install FortiClient

sudo apt install forticlient 

On Linux the forticlient is split into two parts: forticlient and forticlient vpn. You need the vpn part. Note that both applications are called "forticlient" and there is much confusion. On the forticlient download page there is a link to the vpn and another link that takes you to instructions for using a repo. Us the vpn one and download a deb then use the standard install process.