How to send a Post body in the HttpClient request in Windows Phone 8?

I have written the code below to send headers, post parameters. The problem is that I am using SendAsync since my request can be GET or POST. How can I add POST Body to this peice of code so that if there is any post body data it gets added in the request that I make and if its simple GET or POST without body it send the request that way. Please update the code below:

HttpClient client = new HttpClient();

// Add a new Request Message
HttpRequestMessage requestMessage = new HttpRequestMessage(RequestHTTPMethod, ToString());

// Add our custom headers
if (RequestHeader != null)
{
    foreach (var item in RequestHeader)
    {

        requestMessage.Headers.Add(item.Key, item.Value);

    }
}

// Add request body


// Send the request to the server
HttpResponseMessage response = await client.SendAsync(requestMessage);

// Get the response
responseString = await response.Content.ReadAsStringAsync();

UPDATE 2:

From @Craig Brown:

As of .NET 5 you can do:
requestMessage.Content = JsonContent.Create(new { Name = "John Doe", Age = 33 });

See JsonContent class documentation

UPDATE 1:

Oh, it can be even nicer (from this answer):

requestMessage.Content = new StringContent("{\"name\":\"John Doe\",\"age\":33}", Encoding.UTF8, "application/json");

This depends on what content do you have. You need to initialize your requestMessage.Content property with new HttpContent. For example:

...
// Add request body
if (isPostRequest)
{
    requestMessage.Content = new ByteArrayContent(content);
}
...

where content is your encoded content. You also should include correct Content-type header.

I implemented it in the following way. I wanted a generic MakeRequest method that could call my API and receive content for the body of the request - and also deserialise the response into the desired type. I create a Dictionary<string, string> object to house the content to be submitted, and then set the HttpRequestMessage Content property with it:

Generic method to call the API:

    private static T MakeRequest<T>(string httpMethod, string route, Dictionary<string, string> postParams = null)
    {
        using (var client = new HttpClient())
        {
            HttpRequestMessage requestMessage = new HttpRequestMessage(new HttpMethod(httpMethod), $"{_apiBaseUri}/{route}");

            if (postParams != null)
                requestMessage.Content = new FormUrlEncodedContent(postParams);   // This is where your content gets added to the request body


            HttpResponseMessage response = client.SendAsync(requestMessage).Result;

            string apiResponse = response.Content.ReadAsStringAsync().Result;
            try
            {
                // Attempt to deserialise the reponse to the desired type, otherwise throw an expetion with the response from the api.
                if (apiResponse != "")
                    return JsonConvert.DeserializeObject<T>(apiResponse);
                else
                    throw new Exception();
            }
            catch (Exception ex)
            {
                throw new Exception($"An error ocurred while calling the API. It responded with the following message: {response.StatusCode} {response.ReasonPhrase}");
            }
        }
    }

Call the method:

    public static CardInformation ValidateCard(string cardNumber, string country = "CAN")
    { 
        // Here you create your parameters to be added to the request content
        var postParams = new Dictionary<string, string> { { "cardNumber", cardNumber }, { "country", country } };
        // make a POST request to the "cards" endpoint and pass in the parameters
        return MakeRequest<CardInformation>("POST", "cards", postParams);
    }