If you need to validate form fields use server side validation.
Add a custom URL or a hosted API endpoint to the form summary page.
The endpoint will be called on submit prior to submission and any submission events.
If the validations fails the user will be shown a failure modal. This can be used to include an informative error message explaining why the form failed to submit.
If the validation succeeds the form will be submitted as normal.
For an example of Server Side Validation check our our GitHub link here.