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I'd suggest inspecting the response from the request that's sent when you click on the "Verify" button. We had trouble too, but inspecting the response gave us a little more information (here's an example of a response we got - with some info edited out. In this particular case we had a 301 redirect that Apple wasn't keen to follow.) -{ "responseId": "...", "resultCode": ..., "resultString": "Verification failed for domain", "userString": "Error occurred while trying to access 'https://.../.well-known/apple-developer-domain-association.txt'. Received HTTP error code '301'.", "creationTimestamp": "...", "protocolVersion": "...", "userLocale": "en_US", "requestUrl": "https://developer.apple.com/services-account/.../account/ios/identifiers/verifyDomain", "httpCode": 200}

+1 - we would really appreciate this endpoint being documented.We have a variety of use cases that the JS SDK is impractical for, so it would be really helpful for developers working hard on getting Sign In with Apple into their apps to have this endpoint documented.w.r.t `code` - the use of request_mode=form_post means that parameters will be sent back in the body of the POST request. If you haven't updated your callback to accept these parameters from the POST body (rather than as URL parameters), that'll need to be done.