test.php

<?php

function fwp_sortby_views( $options, $params ) {
    $options['views'] = array(
        'label' => 'Most Views',
        'query_args' => array(
            'orderby' => 'post_views',
        )
    );
    return $options;
}
add_filter( 'facetwp_sort_options', 'fwp_sortby_views', 10, 2 );