add_filter( 'wp_nav_menu_items', 'main_menu_logout_link', 10, 2 );

function main_menu_logout_link( $items, $args ) {

    if ($args->theme_location == '' && $args->menu == 'mainmenu') {
        if (is_user_logged_in()):
            $url = wp_logout_url(get_permalink());
              $items .= '<li class="logout-button"><a href=" ' . $url . '">Logout</a></li>';
         endif;
    }
    return $items;
}