Hi Nihal
There are more possibilities, but it depends of your environment and conditions
- you can Import the FG ca cert to your local certificate store (trusted ca store)
- use Firefox and create an exeption 
- you can create a csr and purchase and import a signed ssl certificate (IP or namebased)
https://docs.fortinet.com/d/fortigate-how-to-purchase-and-import-a-signed-ssl-certificate
- you can import (if you have) an exisiting wildcard certificate (needs a dns record for the FG)
Good luck
Markus
