index.ts

const customToken = await admin
  .auth()
  .createCustomToken(device.data.uid, { deviceId: device.id });

const idTokenResponse = await axios.post<SignInWithCustomTokenResponse>(
  "https://identitytoolkit.googleapis.com/v1/accounts:signInWithCustomToken?key=" +
    WEBAPP_APIKEY,
  { token: customToken, returnSecureToken: true }
);
const idToken = idTokenResponse.data.idToken;