Install FortiNet VPN Client

With Ubuntu 120 we do like this to add a repo.
deb [arch=amd64] Index of /repo/6.4/ubuntu /bionic multiverse

How do we do with Zorin Snap
I did manually edit my sources.list

(FortiClient Linux Product Downloads Information)

Sorry not sure I understand your post but with the latest Zorin you are on Ubuntu 20.04LTS, and if you want to install the Fortinet Client then these commands in Terminal should do it:

Ubuntu 18.04 LTS and 20.04 LTS

Install gpg key

wget -O - | sudo apt-key add -

Add the following line in /etc/apt/sources.list (You'll need to open this as Root I believe)

deb [arch=amd64] Index of /repo/6.4/ubuntu /bionic multiverse

Update package lists

sudo apt-get update

Install FortiClient

sudo apt install forticlient