This repository leverages the cloudflare-turnstile-app-check-provider Firebase extension, and offers an implementation of a CustomProvider using Firebase App Check tokens.

const app = initializeApp(firebaseConfig);

const siteKey = 'YOUR-SITEKEY';
const HTTP_ENDPOINT = '${function:function-name.url}';

const cpo = new CloudflareProviderOptions(HTTP_ENDPOINT, siteKey);
const provider = new CustomProvider(cpo);

initializeAppCheck(app, { provider });

Note: the firebase function must be using the aforementioned extension.