Iptables equivalent command in OSX
The "equivalent" of iptables on macOS is pf (packet filter). Refer to the manual page for the configuration file here:

https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man5/pf.conf.5.html

In this case you want to use the "tos" option, as this is the way you set the QoS parameters similar to your --set-dscp option for iptables.

The configuration line could look something like this:

scrub out on $WAN proto tcp set-tos 0x10