calling an endpoint to get cookies

Question

I am developing an application that calls an endpoint, on which I have to authenticate to get cookie[connect. ssid] and use that cookie each I will be POSTing some data,
problem is I tried but with no success at all.

I manually login to that page and got the cookies manually and put in my request as below, and it worked.

The problem is that cookies expire after a couple of requests, my application will be deployed and that will be a huge problem.

I want that process to automatic,

1. request cookies

2. use it if still valid, if not request a new then proceed with the request

var url = "https:abcd";
var httpRequest = (HttpWebRequest)WebRequest.Create(url);
httpRequest.Method = "POST";
httpRequest.Headers["Cookie"] = "connect.sid=s%3AhM62pU-YeAJFQxlNuFxYS76Fy8keCzbO.rECFaOPEiK%2FoVDzbMQjXyYlNsuT0UEew%2FWGlC4An%2FZY"; //value of the cookies
httpRequest.ContentType = "application/json";

using (var streamWriter = new StreamWriter(httpRequest.GetRequestStream()))
{
    streamWriter.Write(carrierjson);
}
var httpResponse = (HttpWebResponse)httpRequest.GetResponse();
using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
{
    var result = streamReader.ReadToEnd();
}
HttpStatusCode statusCode;
statusCode = httpResponse.StatusCode;
if (statusCode == HttpStatusCode.OK)
{
    //somecode here
}

I tried this

//REST API RestSharp but not working

var client = new RestClient("https://xxxxxxxxxxxxxx");
client.Timeout = -1;
var request = new RestRequest(Method.POST);
request.AddHeader("Content-Type", "application/json");
request.AddHeader("Accept", "application/json");
var body = @"{" + "\n" +
@"  ""username"": ""username""," + "\n" +
@"  ""password"": ""password""" + "\n" +
@"}";
request.AddParameter("application/json", body,  ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);