private function setUpCredentials($credentials)
    {
        if (is_array($credentials)) {
            // using full credentials
            $this->clientId = $credentials['client-id'];
            $this->clientSecret = $credentials['client-secret'];
            $this->redirectUrl = $credentials['redirect-url'];
            $this->state = $credentials['state'];
            $this->scopes = $credentials['scopes'];
        } else {
            // using self issued access token
            $this->accessToken = $credentials;
        }
    }