I think I got it. This look OK?
admin@3p-firewall# show firewall group network-group LAN_NETS network 192.168.1.0/24 network 192.168.2.0/24
admin@3p-firewall# show firewall modify modify WAN_LB { rule 10 { destination { group { network-group LAN_NETS } } modify { table main } } rule 20 { modify { lb-group LB } } }
admin@3p-firewall# show load-balance group LB { interface eth0 { } interface eth1 { } }
admin@3p-firewall# show interfaces ethernet eth2 address 192.168.1.1/24 description LAN duplex auto firewall { in { modify WAN_LB } } poe { output off } speed auto
admin@3p-firewall# show interfaces switch switch0 address 192.168.2.1/24 description WLAN firewall { in { modify WAN_LB } } mtu 1500 switch-port { interface eth3 { } interface eth4 { } vlan-aware disable }