Google Authentication - How to retrieve the session id of the newly create session?

I am creating a new user using the google authentication ,
but I have a problem when creating a new session for this user I can’t find the session_id of this session ,
How can I retrieve it please ?
Here is the code I am using :

            using (var httpClient = new HttpClient())
            using (var request = new HttpRequestMessage(new HttpMethod("POST"), "http://localhost:4985/user/_session"))
                request.Headers.TryAddWithoutValidation("accept", "application/json");

                request.Content = new StringContent("{ \"name\": \"Maria\", \"ttl\": 180}");
                request.Content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

                var response = await httpClient.SendAsync(request);
        return "";

Thank you .