Adding a MsgBox when criteria are true

I have a a form that contains 2 text boxes that are not used every time the
form is used. However, if the first box is used the second must be used. So
do I create a MsgBox to pop up based on an if then equation(?)?
The end result would be that when the user exits the second text box
(OnExit) it would look at the previous box and determine if the default value
($0.00) was changed (always a 0 amount). If yes, and the exited text box
is still empty, then put up a pop up to remind that it must be filled in.
Does that makes sense or is there an easier way?
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Thanks and God bless you and yours,
Gabriella777_2