Try to find me!
From the Super Mario Wiki
Mario and co. must travel to Hooktail Castle and use Koops to take the green elevator and go to the fifth floor, near where Ms. Mowz was encountered. After evading a Dull Bones, Mario must drop down and talk to Koopook, who will give Mario the Special Card. If Koops is present when Mario talks to Koopook, some additional dialogue incurs.