$client = $this->client = new Client([
    'base_uri' => 'https://xxx.xxxxxxxxxx.com/api/',
    'timeout' => 15
]);

$this->requestConfig = [
    'auth' => [
        'xxxx@xx.xxx',
        'xxxxx'
    ],
    'headers' => [
        'cache-control' => 'no-cache',
        'content-type' => 'application/x-www-form-urlencoded'
    ],

];

$response = $this->client->get($url, $this->requestConfig);

$content = $response->getBody()->getContents();

string(66) ""[{"ExternalId":"38","AgencyReference":"45436070356676"}]""