job_bm_application_submit_errors_$id

add_filter('job_bm_application_submit_errors_{application_method_id}', 'job_bm_application_submit_errors_{application_method_id}_29072019');
function job_bm_application_submit_errors_{application_method_id}_29072019($error, $form_data){

if(empty($_POST['custom_form_field'])){

    $error->add( 'custom_form_field', __( '<strong>ERROR</strong>: Field is empty.', 'job-board-manager' ) );
}

    return $error;
}