Hi,
You can register the domain name on the DNS server on the internet, then config a vip on the fortigate,mapping the public ip to your internal server ip address with port forwarding(for example port80 forward to port443).
a config example:
config firewall vip
edit " vip-test"
set arp-reply disable
set extip x.x.x.x(public ip)
set extintf " port1"
set portforward enable
set mappedip y.y.y.y(internal ip)
set extport 80
set mappedport 443
next
end
config firewall policy
edit 3
set srcintf " port1"
set dstintf " port2"
set srcaddr " any"
set dstaddr " vip-test"
set action accept
set schedule " always"
set service " ANY"
set nat enable
next