Can someone tell us more about the methods of POST sending via httpclient

How many sites I haven't looked at, there are still questions. Here's the code, and it doesn't seem to be correct

 string loginURI = "https://www.avito.ru/moskva/avtomobili/hyundai/solaris?radius=200#login?s=h";

            var client = new HttpClient();
            var result = await client.GetStringAsync(loginURI);
            Console.WriteLine(result);
            HttpContent content = new StringContent("https://www.avito.ru/api/1/stats/frontend/fps HTTP/1.1");
           var a=  await client.PostAsync(loginURI, content);

It seems that I write the link and http myself, but in other examples I saw this

            var person = new Person ();
            person.Name = "Джон Доу";
            person.Occupation = "садовник";

            var json = JsonConvert.SerializeObject (person);
            var data = new StringContent (json, Encoding.UTF8, "application / json");

            var url = "https://httpbin.org/post";
            использование var client = new HttpClient ();

            var response = ожидание клиента. PostAsync (url, data);

Do I have to send json data? Or what, can you point out the errors and explain what and how?