Laravel singleton doesn't work

So i put this code on the method register() inside the AppServiceProvider:

public function register()
{
    $this->app->singleton(GuzzleHttp\Client::class, function($app) {
        return new Client([
            'base_uri' => env('API_BASE_URI'),
            'headers' => ['X-API-Key' => env('API_KEY')],
            'curl' => [
                CURLOPT_SSL_VERIFYPEER => false
            ]
        ]);
    });
}

but when i do a new instance it simply doesn't work, i know that it doesn't work because on my controller i did:

$client = new GuzzleHttp\Client();

dd($client);

and the debugger page doesnt show the X-API-Key on the headers key that i put on the singleton closure, so, what am i doing wrong?

This is what it shows:

"headers" => array:1 [▼
    "User-Agent" => "GuzzleHttp/6.5.3 curl/7.68.0 PHP/7.4.3"
]

This is what should show:

"headers" => array:1 [▼
    "User-Agent" => "GuzzleHttp/6.5.3 curl/7.68.0 PHP/7.4.3"
    "X-API-Key" => "My API Key Here"
]