DialogReturn

DialogReturn[expr]
closes a dialog window, returning the expression expr from the dialog.

DialogReturn[]
closes a dialog window, returning Null.

If DialogReturn[expr] is used inside a dialog put up by DialogInput, then expr will be the value returned by DialogInput.

If DialogReturn[expr] is used inside a dialog created by CreateDialog, expr will be evaluated and then discarded.

Put up a dialog window that returns 123 when the button is clicked:

Out[2]//FullForm=