https://mega.nz/file/29EyTRjB#ZodVuZh4SIEL6iB3kbzCd-oybmEq84J5Klmg32Y5olw This walkthrough explains and shows exactly where the key is, but it only spawns after you tell the ghost its name. (Check the map for the exact location)
Somewhat unrelated though, but one of the reasons I get the game to tell people to save before the ghost hunt starts, is so that they can reload that save if they get stuck and wanna skip it. Skipping the ghost hunt has no consequences aside from currently making it impossible to unlock the hidden images for it, but that'll likely be changing in future.