Hello,
I found that 'customerid_contact' is a collection-valued navigation property
Associating entities this way using a collection-valued navigation property is not supported by the Web API.
Try this with primarycontactid
{
"casetypecode":2,
title":"Test Case Web aPI",
"primarycontactid" :{
"firstname": "Test Contact with Case with Note",
"lastname": "Web APi Call",
"emailaddress1": "abc@abc.com",
"telephone1": "231497979"
}
}