Using WAN Alias IP for specific hosts in LAN

You need to combine SD-WAN PBR and NAT for this scenario. 
So create a SD-WAN Policy based route for the traffic and select Port2 or Port3. 

Then create a NAT Rule, which uses the needed Source IP for the specific traffic. 

LuCar thank you very much for the quick reply.

but if I were to configure multiple hosts to multiple public IP?

For example:

SERVER:            10.90.90.20                   PUBLIC IP:                  5.88.135.10  

SERVER:            10.90.90.21                   PUBLIC IP:                  5.88.135.11  

SERVER:            10.90.90.22                   PUBLIC IP:                  5.88.135.12  

SERVER:            10.90.90.23                   PUBLIC IP:                  5.88.135.13

How should I set up SD-WAN? what should I set on origin and destination?

Also I need a firewall rule for each server to perform port forwarding of port 443. (rule already done)

This automatically creates 3 NAT rules:

Thanks so much again for the support.

Francesco

If those Server should communicate throught just one interface, you only need one SD-WAN Rule: All Servers, to ANY (or the internetv4 object of this KB https://community.sophos.com/sophos-xg-firewall/f/recommended-reads/129512/xml-import-for-internetv4-objects), using WAN Interface 1. 

Then you need per Server one NAT Rule, which uses server 1 LAN Ip and SNAT it to WAN IP 1. 

Original:  SERVER

Destination:  PUBLIC IP

Service: Any

SNAT: MASQ

DNAT: Original

PAT: Original

Interface IN:  Any

Interface OUT: Any

Correct?

Original:  SERVER

Destination:  PUBLIC IP

Service: Any

SNAT: MASQ

DNAT: Original

PAT: Original

Interface IN:  Any

Interface OUT: Any

Correct?