You can use either HttpClient or RestSharp. Since I do not know what your code is, here is an example using HttpClient:
using (var client = new HttpClient())
{
// This would be the like http://www.uber.com
client.BaseAddress = new Uri("Base Address/URL Address");
// serialize your json using newtonsoft json serializer then add it to the StringContent
var content = new StringContent(YourJson, Encoding.UTF8, "application/json")
// method address would be like api/callUber:SomePort for example
var result = await client.PostAsync("Method Address", content);
string resultContent = await result.Content.ReadAsStringAsync();
}