you could try
"type":"CustomObjectData","ContactID":"'ANYID'","fieldValues":[{.......
POST https://secure.eloqua.com/API/REST/1.0/data/customObject/19
you could try
"type":"CustomObjectData","ContactID":"'ANYID'","fieldValues":[{.......
POST https://secure.eloqua.com/API/REST/1.0/data/customObject/19