RPL is way to go >https://docs.opnsense.org/manual/how-tos/multiwan.html#step-4-policy-based-routingRegards,S.
Do it port based. Create a rule for that port and change the gateway to the preferred WAN
1. Go to FW > Aliases2. Create an Alias type Host(s) name it Battle_net_Server_IP3. Copy in the IPs you want to reach a. save it b. hit apply4. Go to Firewall rules > YOUR LAN interface5. Create a inbound rule with source any(or IP of your host PC) destination Alias Battle_net_Server_IP6. In the Gateway option, choose the Gateway for WAN1 a. saved it7. TestThere are several ways how to approach the rule to be created depends if you only want this specific game shifted to WAN1 or Everything that is related to those destination IPs.https://docs.opnsense.org/manual/aliases.htmlRegard,S.
Check if the traffic is hitting the rule.FW > live view > filter by source your host IP where the game runs.Also the 2nd pictures you showed is what exactly? Looks like window to me.Regards,S.