There is a Quiz event on the super sale Jeju Air event, where you get the prizes are a round trip to korea, a TVXQ model airplane, and mileage. Quiz event runs from 2017.06.26~07.02!

quiz link: here (only on mobiles). It asks you about the super sale event where it is held (=the mobile app), so it is easy to answer~

Below is the poster about the 2017 Super Sale in English for those interested, it does not mention the TVXQ model airplane there though, so perhaps it is intended for Japanese passengers as it is only mentioned in the Japanese post.

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Credits:

Jeju Air Website: 1 and 2 via @snxy,
and the English Official Post,
Post Written and Shared by TVXQ! Express