How to get ID Token in Firebase Cloud Functions

I need to access user.getIdToken() from firebase cloud functions. I am creating a user like so:

exports.addUser = functions.auth
  .user()
  .onCreate((user: admin.auth.UserRecord) =>
    admin.auth()
      .setCustomUserClaims(user.uid, {
        "custom_claims": {
          "USER": user.email,
          "ROLE": 'SUBSCRIBER'
        }
      })
      .then(async () => {
      ...
      // get token
      const token = user.getIdToken();

      // run fetch
      fetch(URL, OPTIONS_WITH_TOKEN);

Obviously user.getIdToken() does not work here, as that is a client method.

createCustomToken does not seem to be the same thing, as it does not contain all the same information (hence it is a custom token), and it adds a claims object instead of inserting the object directly.

How can I get a regular token for a header to run a fetch?

Thanks,

J