Retrieve the HTML of the Old Invoice

var client = new RestClient("https://apitest.nilvera.com/einvoice/Old/22d6b2ca-ba6b-47dc-a07e-846cc0e51a0e/html");
client.Timeout = -1;
var request = new RestRequest(Method.GET);
request.AddHeader("Authorization", "Bearer <API KEY>");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);