VFP 6.0 Updating field values not on form

Pardon the newbie question but I haven't been able to find a answer
searching google or the Foxpro help files:

Suppose you have a wizard built form and you have a few fields you
want to update that are not on the form for user input, because they
are calculated based on the user's input.   I want to write a routine
that will update those fields just before the record is appended or
updated.   Where would I add that code?