Embed custom GTM (Google Tag Manager) (from clients) in Flutter app access

Scenario:
I have a CRM application made in Flutter/Dart that generates a landing page for the web.
Flutter for the web uses CanvasKit to render and sends a javascript code to the client’s browser.

Requirement:
My users need to count the accesses to this landing page using Google Tag Manager.

I need each client/user to be able to track/count landing page accesses in their own Google Tag Manager accounts.
That’s why I need a way for the CRM application made in Flutter to dynamically consider each client’s Tags. That’s why I believe that using a plugin or package may not be the solution.

Problem:
In the backend, I have a text field where I insert the javascript code snippet from Google Tag Manager that the client sent me. So I’m going to retrieve it to use in my Flutter application, but it seems that CanvasKit doesn’t correctly embed the “external” JS (coming from the text field, which is the GTM JS).

Has anyone experienced this issue?