How can I implement tiny.cc post method to get short link in C# winform

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C#

HTTPS

I'm making a small "learning" program to shorten links from the "https://tiny.cc" service.

But I'm new to using the "API" so I'm having a hard time understanding some points
I have searched a lot and tried to read the explanation of the site

https://tinycc.com/tiny/api-docs#shorten



And trying to implement it, but without the correct result
So please help

this is My Code

What I have tried:

using Newtonsoft.Json;
using System;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
   
public static async Task<string> tinyccShortenerAsync(string tinyccAPIKey, string LongURL)
        {
            string ShortURL;

            var payload = new
            {
                long_url = LongURL,
            };

            using (var httpClient = new HttpClient { BaseAddress = new Uri("https://tiny.cc/tiny/api/3/") })
            {
                httpClient.DefaultRequestHeaders.Add("apikey", tinyccAPIKey);

                var body = new StringContent(JsonConvert.SerializeObject(payload), UnicodeEncoding.UTF8, "application/json");

                using (var response = await httpClient.PostAsync("urls/", body))
                {
                    response.EnsureSuccessStatusCode();

                    var link = JsonConvert.DeserializeObject<dynamic>(await response.Content.ReadAsStringAsync());

                    ShortURL = "https://" + link.short_url;
                }
            }

            return ShortURL;
        }

static async Task Main(string[] args)
{

    var task = await tinyccShortenerAsync("TEST_API","http://www.google.com/");

    Console.WriteLine(task);
    Console.ReadLine();

}