Here's the example, thanks!
when LB_FAILED {
set info "client { [IP::client_addr]:[TCP::client_port] -> [clientside {IP::local_addr}]:[clientside {TCP::local_port}] }"
append info " Clock" {[clock format [clock seconds] -format "%d/%m/%Y:%H:%M:%S %z"]}
log local0. "$info - - \[[clock format [clock seconds] -format "%d/%m/%Y:%H:%M:%S %z"]\]"
}
when SERVER_CONNECTED {
set info "client { [IP::client_addr]:[TCP::client_port] -> [clientside {IP::local_addr}]:[clientside {TCP::local_port}] }"
append info " server { [IP::local_addr]:[TCP::local_port] -> [IP::server_addr]:[TCP::server_port] }"
append info " ethernet { [string range [LINK::lasthop] 0 16] -> [string range [LINK::nexthop] 0 16] tag [LINK::vlan_id] qos [LINK::qos] }"
log local0. "$info - - \[[clock format [clock seconds] -format "%d/%m/%Y:%H:%M:%S %z"]\]"
}