Had no clues at first sight. Searched up a bit to figure it out.

> “참코: 애 발매일.”

- **참코**: **참**가<sub>參加</sub> **코**드<sub>code</sub>; passcode required to join.
- **애**: the name of the [Stray Kids](https://en.wikipedia.org/wiki/Stray_Kids)’ digital single “[_Mixtape:**애**_](https://youtu.be/M7lIml8-eHU).”
- **발매일**: 발매<sub>發賣</sub>-일<sub>日</sub>; release date.

> “**Passcode hint** — the release date of the Stray Kids’ digital single ‘Mixtape:애.’”

Which is June 26, 2021. It’s either `210626` or `0626`. There you go.