odata.id property in POST request body

That's strange. Could you give us a real example of it? From my understanding, when a user create a resource, the response should the information of the created resource. For example, when you create an account via /redfish/v1/AccountService/Accounts, the @odata.id in the response body might be "/redfish/v1/AccountService/Accounts/3" according to your uri rule.

Edit:
By the way, there's a perfect example you could refer to in the DSP0266 13.3.4.2 Session login. 

==================================
Request: 

POST /redfish/v1/SessionService/Sessions HTTP/1.1
Host: <host-path>
Content-Type: application/json;charset=utf-8
Content-Length: <computed-length>
Accept: application/json;charset=utf-8
OData-Version: 4.0

{
    "UserName": "<username>",
    "Password": "<password>"
}

==================================
Response: 

HTTP/1.1 201 Created
Location: /redfish/v1/SessionService/Sessions/1
X-Auth-Token: <session-auth-token>

{
  "@odata.id": "/redfish/v1/SessionService/Sessions/1",
  "@odata.type": "#Session.v1_0_0.Session",
  "Id": "1",
  "Name": "User Session",
  "Description": "User Session",
  "UserName": "<username>",
  "Password": null
}