DialogProlog is an option for Dialog which can give an expression to evaluate before the dialog starts.
You must use a delayed rule of the form DialogProlog :> expr to prevent expr from evaluating prematurely.
Expressions given by DialogProlog are evaluated after symbol values are localized, and before any expression given as the argument of Dialog is evaluated.
See The Mathematica Book: Section 2.13.2.
See also: $Epilog.