Lets get started on the setup.
Lync uses port 8080 and 4443 for external web services, all we need to do is do a port forward on the Fortigate.
For this we are going to create a new "Virtual IP"
The External IP Address will be the public IP Address you plan to use to publish your Lync web services on the Internet.
The Internal IP Address will be the internal IP Address of your Lync Front End Server
As a best practice for myself I create another VIP for port 80/8080. so if any user just types dialin.domain.com they will be automatically redirected to the https.
Once you have your 2 VIPs created now to publish them in the Fortigate Policy.
Source Interface: WAN1 or WAN2 depending on which interface you are using to publish Lync.
Destination Interface: Depending on how you have your Fortigate configured. if its in Interface mode you will only have "Internal". If in "Switch" mode you will have Internal1-> 4-6
Destination Address: choose both your VIPs you created above.
Service: as for the service, alot of people choose HTTP and HTTPS, but there is no need to as you already selected your forwarding ports when you created the VIPs. so choose ANY
Once you have applied your firewall policy, try it out!
Also confirm you have created public DNS entries for dialin and meet to point to the public IP Address you used in your VIPs.