搞定！

using (var httpClient = new HttpClient())
{
    httpClient.DefaultRequestHeaders.TransferEncodingChunked = true;
    using (var ms = new MemoryStream())
    {
        var streamContent = new StreamContent(ms);
        for (int i = 0; i < 100; i++)
        {
            var bytes = Encoding.UTF8.GetBytes("Hello World!");
            ms.Write(bytes);
            ms.Flush();

            var response = await httpClient.PostAsync(uri, streamContent);
            Console.WriteLine($"Requested {response.RequestMessage.RequestUri}...{response.StatusCode}");
        }
    }
}