Hey @i_am_sahil,
Apparently there was a mistake in the doc and actually you cannot use “hs_object_id” in the upsert endpoint. This is because this endpoint would create a contact if the id does not match, but the “hs_object_id” cannot be modified nor created via APIs. Therefore, the record id can only be used for the batch update endpoint. The docs will be updated accordingly.
That being said, you can use either the email property or a custom property (with a unique value) for the upsert endpoint.