Get the HTML of the Old Invoice

var options = new RestClientOptions("https://apitest.nilvera.com")
{
  MaxTimeout = -1,
};
var client = new RestClient(options);
var request = new RestRequest("/earchive/Old/ff618918-6243-4ea4-a880-3c0f645c16b1/html", Method.Get);
request.AddHeader("Authorization", "Bearer <API KEY>");
RestResponse response = await client.ExecuteAsync(request);
Console.WriteLine(response.Content);