firestore - read / write from application without granted security rules read / write access

[OrielResearchCure]

Hi,
I would like to use the cloud_firestore library API to read / write services and prices from / to firestore. The operation is done by the flutter web application and does not (as far as I understand) require granting read / write permission by security rule. This is example for the write operationvar:
collection = FirebaseFirestore.instance.collection('Service_Prices'); var someData = {'key':'val'}; collection.add(someData);
I am unable to write to firestore (permission denied), unless the security rules are granting write permission: 
allow write: if true
I don’t want to open the application to read / write from users. What will be the right way to read / write from/to firestore?

A code sample will be very appreciated.

many thanks,
eilalan