1. Navigate to Firewall | Virtual IPs.
2. Click on the Add button.
3. For Type, select IP Alias:

4. Keep the Interface set to WAN and the Address type set to Single address.
5. In the Address text field, enter a virtual IP for the WAN address (for example, 10.1.1.1/8). Select the appropriate number of bits in the drop-down box for the subnet mask.
6. Enter a description in the Description edit box (for example, WAN virtual IP).
7. Click on the Save button.
8. Click on the Apply Changes button.
9. Navigate to Firewall | NAT.
10. Click on the Outbound tab.
11. In the Outbound NAT Mode section, select Manual Outbound NAT.
12. Click on the Save button, and then click on Apply Changes.
13. You should now have a table with a series of manual outbound NAT rules. Find the Auto created rule – LAN to WAN and click on the Edit icon for the rule.
14. In the Translation section, change the Address to the virtual IP we just created:

15. Click on the Save button.
16. Click on the Apply Changes button.