XG 18. snat binding to specific public ip

I have only WAN interface with multiple public ip addres configured as an alias IP. So.. #PortB, #PortB:1, #PortB:2 etc...

All clients leave the internet via the default snat with the firewall public ip configured in port #PortB.
I would like only an internal server to exit through an ip alias, but when I create the NAT in the "outbound interface" I only have port #PortB and I don't see how to tell it to exit with an ip alias.