How do I create a PIN number for the "Whisper" app?

I downloaded the app called "Whisper" where you can post a picture with a secret or something you want to get off your chest written on it. I'm trying to make my first post to create my account, but I've hit a bump. At the last step, it has a place for me to type the username I want, and under that, it says "Create a PIN" and there's a place to put numbers. What do I do there? I've tried typing in random numbers and something keeps popping up that says "your confirmation doesn't match your PIN. What confirmation are they talking about? Help please? Thank you!