What freddy says is the basics of what you want to do. Since submit buttons can perform one or more functions per click, you gotta have a plan on what needs to be done when the user clicks.
Also, by CUSTOM, do you mean that it looks a certain way you want? If so you'll need to duplicate the instances of it and then get into the GUI elements of Flash to alter its features. This is how people get the functionality of these builtin objects, but with a customized look that fits their project.