add_action( 'cmb2_meta_boxes', 'custom_fields' );

function custom_fields( array $metaboxes ) {
    $metaboxes[REALIA_PROPERTY_PREFIX . 'general']['fields'][] = array(
        'id'    => REALIA_PROPERTY_PREFIX . 'custom_field33',
        'name'  => 'Field title',
        'type'  => 'text','position' => 2,

        'show_names'    => true,
        'on_front' => true,
    );

    return $metaboxes;
}