public function refreshAccessToken($refreshToken, $expiresIn = null)
    {
        $parameters = [
            'grant_type' => 'refresh_token',
            'refresh_token' => $refreshToken,
        ];

        if ($expiresIn != null) {
            $parameters['expires_in'] = $expiresIn;
        }

        return $this->tokenRequest('oauth2/token', $parameters);
    }