creating N:N record using odata webapi c# - Bad request error

Hi,

I am trying to create a  record in N:N table via web api, C# (Authentication works fine).

using below code errors out with bad request. Please advice.

JObject prodTypeGUIDObject = new JObject
{
{ "@odata.id", $"/product_types({ProductTypeGUID})" }
};

var url = "">123.dynamics.com/.../$ref";
//var accessToken = await GetAccessToken();
var client = new HttpClient();
var msg = new HttpRequestMessage(httpMethod, url);
msg.Headers.Add("OData-MaxVersion", "4.0");
msg.Headers.Add("OData-Version", "4.0");
msg.Headers.Add("Prefer", "odata.include-annotations=\"*\"");
msg.Headers.Add("Authorization", $"Bearer {crmAccessToken}");
if (body != null)
msg.Content = new StringContent(prodTypeGUIDObject, UnicodeEncoding.UTF8, "application/json");
// return await client.SendAsync(msg);
var result = client.SendAsync(msg).Result;