How to create sign-in service

When I go to create a service for sign in with apple (to use with a webapge) on the Certificates, Identifiers & Profiles page, the configuration is very confusing. It says to click the download button but there isn't one. Where it asks for my domain I enter it and when I hit save it just tells me it not a valid domain...which it definitely is. Is there documentation somewhere showing how to configure this?