how to enforce required field

hello

my problem is this:

there is a required field in a form

when I query the field from DB the field is null

then I can close window with a required feild null

but I need that the form prevented me from closing when the field is null

well, I've tried to set record status  as changed (in PRE-QUERY), so it will force validation

it didn't work

the form asks "do you want to save the changes?" when closing

and then I can choose "Yes" and it "saves" the null value