I don't understand why you want to use forticlient.
The dafault gateway of you wifi client is not a fortigate?
You can use webfilter on fortigate, and create a rule where source is WiFi and destination is internet.
If you want to use Forticlient to block sites ecc, you need to have a management server Forti EMS
In this way you can create a profile where define which sites have to been blocked and which not.
With old version of FortiOS you can define this profile on fortigate, but the new way is Forti EMS that is a dedicate solution for this needs.
With the eval version of EMS you can manage up to 10 devices.