Use HubSpot batch read and batch upsert endpoints with idProperty to read and write records by a custom unique identifier

Steps

1. Identify or create a property on the object that will serve as your external unique key, marking it as hasUniqueValue: true via the Properties API
2. POST to /crm/v3/objects/<objectType>/batch/read with inputs containing id values and the idProperty query parameter set to your property name
3. For upserts POST to /crm/v3/objects/<objectType>/batch/upsert with each input containing idProperty, id (the external key value), and a properties map
4. The upsert creates the record if not found or updates it if the external key matches an existing record
5. Check the results array in the response; each item reports its status (CREATED or UPDATED) and the resulting record
6. Handle 207 multi-status responses by inspecting each individual result for errors alongside successes