I'm trying to setup up web notifications with FCM and I get error messages when trying to register the service worker.

My structure is as follows:

domain.com/notification/index.html

domain.com/notification/test-worker.js

Index.html:

function registerServiceWorker() 
{
    console.log('start register');
    return navigator.serviceWorker.register('/notification/test-worker.js')
    .then(function(registration) 
        {
            console.log('Service worker successfully registered.');
            askPermission();
            return registration;
        })
    .catch(function(err) 
        {
            console.error('Unable to register service worker.', err);
        });
    console.log('end register');
}

test-worker.js

 import { initializeApp } from "https://www.gstatic.com/firebasejs/9.6.1/firebase-app.js";
 import { getAnalytics } from "https://www.gstatic.com/firebasejs/9.6.1/firebase-analytics.js";

 const messaging = getMessaging();
 onBackgroundMessage(messaging, (payload) => {
   console.log(' Received background message ', payload);
   // Customize notification here
  const notificationTitle = 'Background Message Title';
  const notificationOptions = {
   body: 'Background Message body.'
 };

  self.registration.showNotification(notificationTitle,
   notificationOptions);
});

The errors I get are:

Uncaught SyntaxError: Cannot use import statement outside a module
Unable to register service worker. TypeError: Failed to register a ServiceWorker for scope ('https://example.com/notification/') with script ('https://example.com/notification/test-worker.js'): ServiceWorker script evaluation failed

I understand that import cannot be used outside of a module but how can I specify that the service worker should be a module? I have seen many examples of service workers using import so I guess it should be possible to use import. Or should I use a different method?

Thanks