I'm new to their API too, but searching around it seems like you actually want the POST version batch-get endpoint, from either the BIM360 API or the ACC API, not the Data Management API.
Xiaodong Liang posted an article about it here: https://aps.autodesk.com/blog/custom-attributes-apis-bim-360-document-are-now-public-beta.
He also has a related question/answer here: Not able to get custom attribute data from Autodesk BIM360 for data management API