How to prevent a form submission on a CiviCRM form with jquery?
I would add validator method via js which would stop the submit until the validator returns true
CRM.$(function($) {
  $.validator.addMethod('concessionaryError', function(value, element) {
    let isValid = true;
    if (value == '') {
      isValid = false;
    }
    return isValid;
  });
  $('#custom_39').addClass('concessionaryError valid');
  $.extend($.validator.messages, {concessionaryError:'please correct the error'});
});

HTH
Pradeep