It seems you must first have an *OS app in order to use Apple Sign In on the web, a $100/year barrier to entry for web developers verses Google/Facebook auth.

"To configure web authentication, you must create a Services ID and associate your website to an existing primary iOS, macOS, tvOS, or watchOS App ID enabled for Sign In with Apple."

Source: https://help.apple.com/developer-account/#/dev1c0e25352