You can use the "host-check" function for this.
When the client connects to the firewall, the firewall sends out a check to the VPN client to look for:
1. Registry string
2. A file on your computer
3. A running process.
4. If you have a firewall software
5. If you have a antivirus software
So for your problem, use option 1,
config vpn ssl web host-check-software
edit <a name>
set type registry
set target <your registry string>
(be sure to type "get" and see all available options)
Then associate this policy with your SSL VPN portal.
FortiGate 200A/B, 224B, 110C, 100A/D, 80C/CM/Voice, 60B/C/CX/D, 50B, 40C, 30B
FortiAnalyzer 100B, 100C
FortiAP 220B/221B, 11C