I had a very difficult time using a similar 20% off e-cert on an international itinerary. The only way I was ultimately able to use the percentage-off e-cert was to call the phone number listed for united.com support, get connected to a reservations agent in India, immediately ask to speak with a supervisor (I learned after my second futile attempt to book travel that only a supervisor was able to ticket using the e-cert), who, after many delays and times being placed on hold, was ultimately able to apply the e-cert. Unfortunately, UA has made it very difficult to use %-off e-certs on international itineraries.

Good luck.