you can see this documentation to help you about it.
Mapping a specific IP address to another specific IP address is usually called Destination NAT (DNAT). When this central NAT table is not used, FortiOS calls this a Virtual IP address (VIP). DNAT, or VIP, is used to map an external IP address to an IP address or address range. The mapping can include all TCP/UDP ports or, if port forwarding is enabled, it only refers to the specific configured ports. As the central NAT table is disabled by default, the term VIP is usually used.
VIPs are typically used to NAT external or public IP addresses to internal or private IP addresses. Using a VIP between two internal interfaces made up of private IP addresses is possible, but rare, because the two networks can just use the IP addresses of the networks without any address translation. Using a VIP for traffic going from the inside to the internet is supported, but unlikely to be required.
To apply a virtual IP to policy using the CLI:
config firewall policy
edit 8
set name "Example_Virtual_IP_in_Policy"
set srcintf "wan2"
set dstintf "wan1"
set srcaddr "all"
set dstaddr "Internal_WebServer" --->>> this is a DNAT
set action accept
set schedule "always"
set service "ALL"
set nat enable
next
end
https://docs.fortinet.com/document/fortigate/6.2.10/cookbook/510402/static-virtual-ips
