C# use proxy with HttpClient request

Posting a JSON string to server using a proxy

var httpClientHandler = new HttpClientHandler
{
    Proxy = new WebProxy("http://localhost:8888", false),
    UseProxy = true
};
using (var client = new HttpClient(httpClientHandler))
{
    string apiUrl = "http://yourserver.com/postpage";
    StringContent content = new System.Net.Http.StringContent("{dataelem:value}", Encoding.UTF8, "application/json");
    HttpResponseMessage response = client.PostAsync(apiUrl, content).Result;
}