public function getToken()
{
    $this->token = '';
    try {
        $response = $this->client->request('POST', 'auth/login', [
            'form_params' => [
                'email'    => $enlazatteUser,
                'password' => $enlazattePassword
            ]
        ]);

        $responseData  = json_decode($response->getBody()->getContents());

        $this->token   = $responseData->token;
        $this->message = 'Token correcto.';
    } catch (RequestException $e) {
        $statusCode = $e->getResponse()->getStatusCode();

        switch ($statusCode) {
            case 403:
                $this->message = 'Error de credenciales al obtener token.';
                break;
            case 422:
                $this->message = 'Error, request invalido.';
                break;
        }
    }

    return $this->token;
}

{
    "status": "ok",
    "token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9..."
}

                            
$("#register-form").validate({
  rules: {
    username: {
      required: true,
      minLength: 2,
      remote: "register.php"
    }
  },
  messages: {
    username: {
      required: "Enter your username",
      minLength: "At least 2 characters are necessary",
      remote: String.format("The name {0} is already in use")
    }
  }
});