.NET (in C#)

using System.Net;

class Demo {
  static async Task Main(string[] args) {
    string username = Uri.EscapeDataString("{PROXY_USERNAME}");
    string password = "{API_Key}";
    string url      = "https://cloudflare.com/cdn-cgi/trace"; // Insert your target URL here
    string proxy    = "http://isp.joinmassive.com:4080";

    HttpClientHandler   handler  = new() {
                                     Proxy                   = new WebProxy(proxy),
                                     DefaultProxyCredentials = new NetworkCredential(
                                                                 username, password
                                                               )
                                   };
    HttpClient          client   = new(handler);
    HttpResponseMessage response = await client.GetAsync(url);

    Console.WriteLine(await response.Content.ReadAsStringAsync());
  }
}