Hello,
You need to implement the web-service yourself. See
https://rfc5766-turn-server.googlecode.com/svn-history/r278/docs/TURNServerRESTAPI.pdf for details of how the web-service should work. It is worth noting that this document has been updated (and the format of the credentials for use with auth_ephemeral has changed in it) since auth_ephemeral was implemented (and is now available as an IETF draft too). The module will be updated to support the latest version soon.
The point of auth_ephemeral is to give you a way to authenticate with SIP (or MSRP or HTTP) on Kamailio based on credentials received from a web-service. But what this web-service is, and how you authenticate with it in the first place, depends on your application.
Regards,
Peter