https://*Blacklisted site*/d/14463-fc2-discussions-and-requests/268
seems like its one of those premium site scams, hopefully not.
anyways, can you try using the site name as the password? try without capitalization, with first letter cap, with and without .com, etc.
edit: scratch that -not a premium scam site i think. After checking the site, it seems like the password is set by the original uploader on fc2, which is then sent after purchase. Some of the stuff uploaded on kawaiijk doesnt have password, while the ones that has a password is explicitly written on the post for older works. (see
this as an example). I'll do some further readings on it.