Although I haven’t found a way to do it via the web interface QoS, manually entering it on the shell works.
root@zeroshell root> iptables -t mangle -A FORWARD -p tcp --dport 12345 -j DSCP --set-dscp 1
root@zeroshell root> iptables -t mangle -L -v
Chain PREROUTING (policy ACCEPT 12M packets, 1040M bytes)
pkts bytes target prot opt in out source destination
[...]
Chain INPUT (policy ACCEPT 1198K packets, 92M bytes)
pkts bytes target prot opt in out source destination
[...]
Chain FORWARD (policy ACCEPT 11M packets, 948M bytes)
pkts bytes target prot opt in out source destination
[...]
0 0 DSCP tcp -- any any anywhere anywhere tcp dpt:italk DSCP set 0x01