If you want to reply to a thread, view the thread and at the top of the screen there is a box for you to fill out your name, email, comment, upload a file, etc.

If you want to start a thread, fill out this box when viewing the forum, not a specific thread.

you can leave everything blank except for comment. but imo be sure to write your full first and last name, and a valid email address. especially when posting CP.