You should add the authorisation headers in the Client configuration such as:

$client = new GuzzleHttp\Client([
    'base_uri' => '127.0.0.1:3000',
    'headers' => [
        'X-API-Key' => 'abc345'
    ]
]);

See: https://docs.guzzlephp.org/en/stable/request-options.html#headers