I have a UserControl where I have some buttons and textboxes. I was wondering how I can display that UserControl when a user clicks a button.
A user control cannot be a 'popup window', that requires a toplevel window. A form. You can put the user control in a form and use the form's Show() method to make it visible.
Fwiw, turning a user control into a toplevel window is technically possible with the SetTopLevel() method. It isn't worth the hassle, it won't behave like a proper one.