Sorry, I mis-read your question. (My brain's not properly "in gear" today.)
I'm afraid I can't help you there. I don't think Scribus has any functionality in that area. I could be wrong though.
Would you really want to require field validation on save anyway?
For example, what would happen if someone was filling in the form but couldn't remember some details and wanted to save it until they could get the info? If you made that field mandatory before you could save the form and the reader didn't have that information to hand then they couldn't save the form. So they'd have to discard all of their changes and fill it all in again later. Not very user friendly.
I'd say that you'd be better putting the validation on a button instead of forcing it on save. That's just my opinion though.
Maybe someone else can help you better.