public function getAccessCredentials($code, $state = null)
    {
        return $this->post('oauth2/token', [
            'client_id' => $this->getClientId(),
            'client_secret' => $this->getClientSecret(),
            'grant_type' => 'authorization_code',
            'redirect_uri' => $this->getRedirectUri(),
            'code' => $code,
            'state' => $state,
        ]);
    }