I don't know what you are trying to do with the custom class, but you do have the ability to rename the class level on the Start sheet bottom of section 2, so you could pick the class the best fits your custom class in terms of hit dice, spell ability, etc. Then create a custom subclass with the details you want to change.
Well, meaning that there's no way to enter a custom home-brew kind of class. For instance, there was a class made called the Blood Hunter with some sub-classes within that. You have the option to have custom sub-classes, but not base classes.