Tkinter- Building a message box

[Tuvas]

I've been trying to build a fairly simple message box in tkinter, that
when a button is pushed, will pop up a box, that has a line of text, an
entry widget, and a button, that when the button is pushed, will return
the value in the line of text. However, while I can read the value of
the button, I want to wait till the button is pushed to return the
value. Any ideas of how I could do this?