Cookie missing in Restsharp response

Question

I use Restsharp for a WPF client that I'm developing.

It appears that I receive no cookies in the client.CookieContainer where it has always 0 items after I successfully authenticate to our server.

It's weird because the same request is sent with Postman and I receive a JSESSIONID cookie that is not present when the request is sent with Restsharp.

public static async Task<IRestResponse> SendLogonRequest(string UID, SecureString pwd)
    {
        var restClient = new RestClient(new Uri(URLSRV))
        {
            Authenticator = new HttpBasicAuthenticator(UID, pwd.ToInsecureString())                //base64 auth;
        };
        restClient.CookieContainer = new CookieContainer();
        var restRequest = new RestRequest(Method.POST);
        restRequest.AddHeader("Accept", "application/json");
        restRequest.AddHeader("Content-Type", "application/json");

        var cancellationTokenSource = new CancellationTokenSource();
        var restResponse = await restClient.ExecuteTaskAsync(restRequest, cancellationTokenSource.Token);

        return restResponse;
    }

I've seen on a post that if the cookie has a HttpOnly flag it will not work. (see: https://stackoverflow.com/a/21072840/7031019) this doesn't help me because I can't change anything from server side.

Thank you