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unfortunately, for now the app_instance_id is not available for web. please read the full explanation in Github from Firebase Analytics repo maintainer in here
and for now, from the event builder measurement procotol web, it seems that we can’t use Firebase for web client, we have to use gtag instead. Firebase is only for mobile. the JSON body and query parameters will be different if you send event via gtag or via Firebase
https://ga-dev-tools.google/ga4/event-builder/
this is the documentation to send event via gtag
Source:stackexchange.com