Re: create a rule for an https
sice that is an url this can only be done using UTM.
URL Filter is your Friend.
So add a policy that allows internettraffic using url filter and make sure that it comes before your other internet blocking policy. Add a url filter to this new policy and set urlfilter in this order:
url you want to alow - exempt
So will allow your url and stop filtering the packet here (=exempt) and block any other url.
Any traffic that does not match the new policy but goes to the internet will stil hit your other internet blocking policy.
This will require a valid Fortinet UTM license on your FGT of course.