var options = new RestClientOptions("https://apitest.nilvera.com")
{
MaxTimeout = -1,
};
var client = new RestClient(options);
var request = new RestRequest("/evoucher/Send/Xml/Preview", Method.Post);
request.AddHeader("Authorization", "Bearer APIKEY");
var body = @"";
request.AddParameter("text/plain", body, ParameterType.RequestBody);
RestResponse response = await client.ExecuteAsync(request);
Console.WriteLine(response.Content);