Go to the transit counter airside, or the ticket counter outside of security. Tell them you want to list as a jumpseater / OMC. If you're at the ticket counter, sometimes you'll need to look for a supervisor before someone knows how to do it, but they eventually muddle through. I've had 50/50 luck on them charging me departure tax or not, usually it's less than $40. Always be ready to book a zed ticket if you get stonewalled, although I've had luck out of all the Asian gateways, even CTU.