Cannot send a content-body with this verb-type using streamWriter & json

Question

I am testing sending some data to an api. The demo code on their site is copied below (the address and data I have changed as its a clients data).

On the line, using (var streamWritier...) I get the error below,

Cannot send a content-body with this verb-type

I have not seen data sent this way before. Is there something I am missing?

using System;
using System.IO;
using System.Net;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            var authStr = System.Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes(@"username:password"));

        var httpRequest = (HttpWebRequest)HttpWebRequest.Create("https://some-api");

        httpRequest.Headers.Add("Authorization", "Basic " + authStr);
        WebResponse response = null;

        try
        {
            httpRequest.ContentType = "application/json";
            httpRequest.Accept = "application/json";
            string data = "{  \"RequestType\": \"ClientAdhoc\",  \"IdentifierType\": \"KDF\",  \"ReqDate\": 20190101"}";
            using (var streamWriter = new StreamWriter(httpRequest.GetRequestStream()))
            { streamWriter.Write(data); streamWriter.Flush(); streamWriter.Close(); }

            response = httpRequest.GetResponse();
            var responseStream = response.GetResponseStream();
            var reader = new StreamReader(responseStream);
            Console.WriteLine("Response received: [{0}].", reader.ReadToEnd());
        }
        catch (Exception exception)
        {
            Console.WriteLine("Exception occurred: [{0}].", exception.Message);
            Console.WriteLine(exception.StackTrace);
        }
       }
     }
    }