Communicating from/to usercontrol in tabcontrol in form

Communicating from/to usercontrol in tabcontrol in form

I thought C# was hard. Try posting a question in stackoverflow.
I have a listbox and a button in a usercontrol, itself in a tabpage of a
tabcontrol, itself on a form. I need to populate the listbox from the form
when the button is clicked.
form > tabcontrol > tabpage > usercontrol > listbox & button
So, how do you notify the form that a deeply buried button has been
clicked and then fill the listbox from the form (or call the usercontrol
from the form to populate the listbox)?
Thank you guys.