I think you still have to specify the URL up to the greader.php page, maybe in your case it would be
https://freshrss.example.com/api/greader.php
A beautiful RSS client finally adds support FreshRSS, my self hosted RSS aggregator of choice, and also for the gReader API that plenty of other services use. The dev notes however that at this time Inoreader is not supported via gReader API.
I think you still have to specify the URL up to the greader.php page, maybe in your case it would be
https://freshrss.example.com/api/greader.php