Search only within the current category (or any taxonomy) archive

This code will restrict the search result to the currently active taxonomy term archive.

Change the $taxonomy variable on line 3 for which taxonomies the code should apply to.

What is this, and where do I put this custom code?

add_action('asp_layout_in_form', 'asp_layout_in_form_archive_input', 10);
function asp_layout_in_form_archive_input() {
    $taxonomies = 'category, post_tag, product_cat'; // Enter the taxonomy names here

    // --- DO NOT CHANGE ANYTHING BELOW THIS LINE ---
    $taxonomies = explode(',', $taxonomies);
    foreach ( $taxonomies as $k => &$tax )
        $tax = trim($tax);
    if (
        ( in_array('category', $taxonomies) && is_category() ) ||
        ( in_array('post_tag', $taxonomies) && is_tag() ) ||
        is_tax($taxonomies)
    ) {
        $obj = get_queried_object();
        if ( isset($obj, $obj->term_id) ) {
            ?>
            <input type="hidden" styl="display:none;" name="asp_tax_archive" value="<?php echo $obj->term_id; ?>">
            <?php
        }
    }
}
// --- DO NOT CHANGE ANYTHING BELOW HERE EITHER ---
add_filter( 'asp_query_args', 'asp_archive_page_category_restriction', 10, 1 );
function asp_archive_page_category_restriction($args) {
    if ( isset($_POST, $_POST['options']) ) {
        parse_str($_POST['options'], $so);
        if ( !empty($so['asp_tax_archive']) ) {
            $term = get_term($so['asp_tax_archive']);
            if ( !is_wp_error($term) ) {
                $args['post_tax_filter'][] = array(
                    'taxonomy' => $term->taxonomy,
                    'include'  => array($term->term_id),
                    'exclude'  => array(),
                    'logic'    => 'AND',
                    'allow_empty' => false
                );
            }
        }
    }
    return $args;
}