You don't.
Instead, you create an event as part of your UserControl which the control raises when the button Click happens, and the outside world handles that.
That way, the outside world doesn't need to know anything about the controls internals, and the control is free to change the way it works without affecting the outside world.
Events are easy to create:
A Simple Code Snippet to Add an Event[
^] shows how to make it even easier!