firebase-auth-2-step-4.dart

@override
void initState() {
  deepLinkSubscription = getLinksStream().listen((String link) {
    loginWithGitHub(link);
  }, cancelOnError: true);
  super.initState();
}

//...

void loginWithGitHub(String link) async {
  //Step 4
  final response = await http.post(
    "https://identitytoolkit.googleapis.com/v1/accounts:signInWithIdp?key=" + FIREBASE_API_KEY,
    headers: {
      "Content-Type": "application/json",
      "Accept": "application/json"
    },
    body: "{\"requestUri\":\"" + link
        + "\"}",
  );
  
  final AuthCredential credential = GithubAuthProvider.credential(
    json.decode(response.body)['oauthAccessToken'],
  );
  //...
}