how to pass json object as an value for one of the key in post request?

I have three keys personname,billingdate and nozzolereadings...I am able to work with name and date as it is simple string but nozzolereadings is asking for json object as a value..

My required json request is:
personname name
billingdate 2019-01-01
nozzolereadings [{"opening_reading":"7687","closing_reading":"7691289"},{"opening_reading":"103250","closing_reading":"103348"}]

Anyone suggest please how to send [{"opening_reading":"7687","closing_reading":"7691289"},{"opening_reading":"103250","closing_reading":"103348"}] as value to post request ?This is my code for sending request to API and getting response...

class Bill
{
public async static Task AddBillData(string name,string date)
{

          Uri uri = new Uri(urlname);

        var postData = new List<KeyValuePair<string, string>>
            {
                new KeyValuePair<string, string>("personname",name),
                new KeyValuePair<string, string>("billingdate",date),


            };

        HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, uri);
        req.Content = new FormUrlEncodedContent(postData);

        using (HttpClient client = new HttpClient())
        {
            var response = await client.SendAsync(req);

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

            statusCheck = JsonConvert.DeserializeObject<StatusView>(content);
        }

        return statusCheck;

    }
}