i cant find them on Askgambler website search, i think duckdice.io is not yet listed there
Probably he is suggesting to use the file a complaint form feature[1] of Askgambler which you are the one who will report the casino based on the specific complaint that you want to be heard. Just click the reference link below to on the askgambler page for reporting complaints. Im sure that they will answer there since they are very active here in the forum.
[1]
https://www.askgamblers.com/submit-complaintyeah tried that but since Duckdice.io is not listed there, i can't select it.