url: /api/data/v8.0/contacts
json:
{
"firstname" : "New",
"lastname" : "Contact",
"[email protected]" : "/accounts(c6da77b6-d53e-e611-80b9-0050568a6c2d)"
}
As the parentcustomerid can be an account or contact you need to define the type of entity you want to set with _entityname after parentcustomerid.