NovaServiceProvider.php
<?php

namespace App\Providers;

use Giuga\LaravelNovaSidebar\NovaSidebar;
use Giuga\LaravelNovaSidebar\SidebarGroup;
use Giuga\LaravelNovaSidebar\SidebarLink;

...

public function tools()
{
    return [
        (new NovaSidebar())
            ->addGroup((new SidebarGroup())
                ->setName('Utilities')
                ->addLink((new SidebarLink())
                    ->setName('Telescope')
                    ->setType('_blank')
                    ->setUrl('/telescope')
                )
                ->addLink((new SidebarLink())
                    ->setName('Horizon')
                    ->setType('_blank')
                    ->setUrl('/horizon')
                )
            )
        ->addLink((new SidebarLink())
            ->setUrl('https://google.com')
            ->setType('_blank')
            ->setName('Google')
        )
    ];
}

OR

public function tools()
{
    return [
        (new NovaSidebar())->hydrate([
            'Utilities' => [
                ['Telescope', '/telescope'],
                ['Horizon', '/horizon'],
                ['Google', 'https://google.com', '_self']
            ],
            'Google 2' => 'https://google.com'
        ])
    ];
}