Prevent Connection to WiFi if Forticlient is not installed on endpoint

If you mean your company's WiFi, then you can use ZTNA tags in the firewall rules that are allowing traffic from the SSID(s) to the other networks.

Once you do that, only clients that have the tags can communicate through the desired SSID(s).

Here is a tutorial.

https://docs.fortinet.com/document/fortigate/7.2.11/administration-guide/477578

Also here is a full ZTNA documentation, in case you want to read more about ZTNA, to understand it more and to be more able to configure it.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-ZTNA-Zero-Trust-Network-Access-Resource-List/ta-p/286196

Hope it helps.