There is no such word as "preregister". There isn't even a concept to which that word might apply.
As far as I know fencing is the only activity in the world where people don't understand what the word "register" means and, therefore, use "preregister".
You don't "preregister" to do something. You register to do it. Then you show up and check in. Perhaps you "preregister" when you find the pen you'll need to fill out the registration form. Or maybe it's when you ask your mom for the registration fee.
So knock it off, or I'll get you.