Answer
Building a correctly-formatted OData batch request by hand can be tricky to get right from documentation alone — a practical technique is to use a list-type page in the actual client that supports adding several new records at once (for example, a Customers list), add a few test records, and save; then inspect the actual request payload the client itself sent, either through browser developer tools (the Network tab) or the platform's own log window. Using that captured real payload as a template for your own external batch request is a reliable way to get the correct batch-request format for creating multiple records in one call, since it's the exact format the client itself already knows how to construct correctly.