How to include a JS file in this theme?

From wp_enqueue_scripts action:

wp_enqueue_scripts is the proper hook to use when enqueuing items that are meant to appear on the front end.

For admin-facing scripts, the correct action hook is admin_enqueue_scripts

function prefix_enqueue_scripts3() {
    wp_enqueue_script(
        'metabox_js',
        get_stylesheet_directory_uri() . '/js/metabox.js',
        array( 'jquery' ),
        '1.0.0',
        true
    );
}
add_action( 'admin_enqueue_scripts', 'prefix_enqueue_scripts3' );