Adding search term to current query (Wordpress PHP) - php

Question

Currently I've got this bit of code to do post-per-page filter to my woocommerce shop
function ps_selectbox() {
$per_page = filter_input(INPUT_GET, 'perpage', FILTER_SANITIZE_NUMBER_INT);
ob_start();
echo '<div class="woocommerce-ordering">';
echo '<select onchange="if (this.value) window.location.href=this.value">';
$orderby_options = array(
'16' => '16 items per page',
'32' => '32 items per page',
'64' => '64 items per page'
);
foreach( $orderby_options as $value => $label ) {
echo "<option ".selected( $per_page, $value )." value='?perpage=$value'>$label</option>";
}
echo '</select>';
echo '</div>';
return ob_get_clean();
}
add_action( 'pre_get_posts', 'ps_pre_get_products_query' );
function ps_pre_get_products_query( $query ) {
$per_page = filter_input(INPUT_GET, 'perpage', FILTER_SANITIZE_NUMBER_INT);
if( $query->is_main_query() && !is_admin() ) {
$query->set( 'posts_per_page', $per_page );
}
}
It will append "?perpage=$value" to the url and change number of items per page in the shop.
But when it comes to search result page, it will clear the current query and replace it with just "?perpage=$value"
EX: "https://aaa.com/?s=C21&post_type=product" to "https://aaa.com/?perpage=16"
If I manually change the url to be ""https://aaa.com/?s=C21&post_type=product&perpage=32" it works.
I tried add_query_arg( $args ) and a condition check but it doesn't seem to work.
EX:
function ps_pre_get_products_query( $query ) {
$per_page = filter_input(INPUT_GET, 'perpage', FILTER_SANITIZE_NUMBER_INT);
if (is_search()) {
add_query_arg( $per_page );
}
elseif( $query->is_main_query() && !is_admin() ) {
$query->set( 'posts_per_page', $per_page );
}
}
But it doesn't seem to work.
Is there anyway I can keep this perpage dropdown feature in search result page?
Thanks in advance. Cheers.