you you can add validation without running form builder.
Go to Webassist -> Forms -> Validation -> Serve validation to add the validation server behavior.
The checkout page ussually post to the confirm page, so the validation needs to be applied to the confirm page.
Before applying the validation, go to the bindings panle on the confirm page, clic kthe plus button and select Form Data, click the folder icon and select the checkout page, this will add dynamic data bindings for the checkout form to the confirm page.