Version 5.x has been out of support since December 13, 2022, and this corresponding section of the documentation is no longer maintained. We strongly recommend you upgrade to the latest supported version of 7.x and read the latest version of this documentation.
You can try Duende IdentityServer with your favourite client library. We have a test instance at demo.duendesoftware.
On the main page you can find instructions on how to configure your client and how to call an API.