Had no clue at first sight. Searched up a bit to figure it out.
“참코: 애 발매일.”
- 참코: 참가參加 joining 코드code; passcode required to join.
- 애: the name of the Stray Kids’ digital single “Mixtape:애.”
- 발매일: 발매發賣 release-일日 date; release date.
“Passcode hint — the release date of the Stray Kids’ digital single ‘Mixtape:애.’”
Which is June 26, 2021. It’s 210626. There you go. Good luck with your K-pop journey.