Then you better register in the app itself. Thus, you will have one account for the application and the site. To do this, open the application and go to the "Settings" menu:

Then choose "Account Details"

Choose the auth method (via Email or Facebook). Fill up nick and password fields. You can either set up an avatar. After that you can use your Email as the login for this site.

If you already registered in app, you can enter this site using Email and password.

Then you can register just for this site. Using your Email.