Can't find how to use HttpContent

Just use...

var stringContent = new StringContent(jObject.ToString());
var response = await httpClient.PostAsync("http://www.sample.com/write", stringContent);

Or,

var stringContent = new StringContent(JsonConvert.SerializeObject(model), Encoding.UTF8, "application/json");
var response = await httpClient.PostAsync("http://www.sample.com/write", stringContent);

To take 6footunder's comment and turn it into an answer, HttpContent is abstract so you need to use one of the derived classes:

For JSON Post:

var stringContent = new StringContent(json, Encoding.UTF8, "application/json");
var response = await httpClient.PostAsync("http://www.sample.com/write", stringContent);

Non-JSON:

var stringContent = new FormUrlEncodedContent(new[]
{
    new KeyValuePair<string, string>("field1", "value1"),
    new KeyValuePair<string, string>("field2", "value2"),
});
var response = await httpClient.PostAsync("http://www.sample.com/write", stringContent);

And if you need to get the response:

var stringContent = await response.Content.ReadAsStringAsync();

https://blog.pedrofelix.org/2012/01/16/the-new-system-net-http-classes-message-content/

While the final version of HttpContent and the entire System.Net.Http namespace will come with .NET 4.5, you can use a .NET 4 version by adding the Microsoft.Net.Http package from NuGet

I'm pretty sure the code is not using the System.Net.Http.HttpContent class, but instead Microsoft.Http.HttpContent. Microsoft.Http was the WCF REST Starter Kit, which never made it out preview before being placed in the .NET Framework. You can still find it here: http://aspnet.codeplex.com/releases/view/24644

I would not recommend basing new code on it.