Had no clues at first sight. Searched up a bit to figure it out.
“참코: 애 발매일.”
- 참코: 참가參加 코드code; passcode required to join.
- 애: the name of the Stray Kids’ digital single “Mixtape:애.”
- 발매일: 발매發賣-일日; 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.