by_stock – a second sorting parameter

Hello,
I want to make a second sorting option by_stock.
I do like this, it does not work(

function sv_change_sorting_fallback( $fallback, $orderby ) {
    if ( 'featured_first' === $orderby ) {
        $fallback = 'by_stock';
    }
    return $fallback;
}
add_filter( 'wc_extra_sorting_options_fallback', 'sv_change_sorting_fallback', 10, 2 );
function sv_change_sorting_fallback_order( $fallback_order, $orderby ) {
    if ( 'featured_first' === $orderby ) {
        $fallback_order = 'DESC';
    }
    return $fallback_order;
}
add_filter( 'wc_extra_sorting_options_fallback_order', 'sv_change_sorting_fallback_order', 10, 2 );

Help me please!)

https://ww.wp.xz.cn/plugins/woocommerce-extra-product-sorting-options/