Advanced Post Filtering by Meta Value

<?php
function filter_posts_by_meta_value($query) {
    if (!is_admin() && $query->is_main_query() && is_post_type_archive('portfolio')) {
        $meta_query = array(
            array(
                'key' => 'custom_meta_key',
                'value' => 'desired_value',
                'compare' => '='
            )
        );
        $query->set('meta_query', $meta_query);
    }
}
add_action('pre_get_posts', 'filter_posts_by_meta_value');
?>