There is no hook that runs your own validation before a submission is saved.
The built-in checks are described in Validation. To run your own rules, listen to formforge/submission_created. It fires after the submission is saved, so it can flag or report an entry but it cannot reject it.
php
add_action( 'formforge/submission_created', function ( $submission_id, $payload ) {
// $payload['fields'] holds the submitted values by field key
if ( empty( $payload['fields'] ) ) {
return;
}
// check them against your own rules and report the result
}, 10, 2 );