It looks like you’re using .Net and RestSharp, correct? Here’s an example that I think will work for you.
// Get these from a config file instead of hard-coding
const string hostName = "yourHostName";
const string token = "yourApiToken";
// Create the client (one instance for the entire app)
var client = new RestClient($"https://{ hostName }.api-us1.com/api/3");
client.AddDefaultHeader("Api-Token", token);
client.AddDefaultHeader("Content-Type", "application/json");
client.AddDefaultHeader("Accept", "application/json");
// create and execute a request
var request = new RestRequest("contacts", Method.GET);
request.AddParameter("status", -1);
var response = client.Execute(request);
Console.WriteLine(response.StatusCode);
Console.WriteLine(response.Content);