Meta Box

Thank you Anh,
I register my menu like this, it displays but the value from setting page doesn't seem to be passed in. am I doing it right?

if ( ! function_exists( 'prefix_theme_setup' ) ) {
	function prefix_theme_setup() {
		// Nav Menus
		register_nav_menus( array(
			'mega-menu' => __( 'Mega Menu', 'textdomain' ) // Mega Menu
		) );
	}
}
add_action( 'after_setup_theme', 'prefix_theme_setup');