Is there a way to make Form Watchers work for external users, or am I missing something?

[yoodima]

I need to implement live notifications when respondents fill out a Google Form. While Google Script Triggers would typically work, they are limited to 10 triggers per user, which is not sufficient for my use case.

To overcome this, I implemented a Form Watcher using Google Cloud Pub/Sub. However, I found that it only works for users within my organization. It does not function for external (third-party) users, regardless of the OAuth scopes I request.

When I make a request to create a Watcher (as described in the official docs), it succeeds if the form belongs to an account within my Google organization.

However, when attempting the same request with a third-party account, I receive the following error:
“Project does not have necessary OAuth permissions from authorizing user.”

Interestingly, if the third-party user shares the form with an account within my organization, and I then make the request on behalf of that internal account, the request succeeds.

But it isn't a good solution since the user have to share his form with another account.

Is there a way to make Form Watchers work for external users, or am I missing something?