A Special Code is Assigned to the Invoice

var options = new RestClientOptions("https://apitest.nilvera.com")
{
    MaxTimeout = -1
};
var client = new RestClient(options);
var request = new RestRequest("/earchive/Invoices/SpecialCode", Method.Put);
request.AddHeader("Content-Type", "application/json");
request.AddHeader("Authorization", "Bearer <API KEY>");
var body = @"
{
    ""UUID"": [
        ""3fa85f64-5717-4562-b3fc-2c963f66afa6""
    ],
    ""SpecialCode"": ""string""
}";
request.AddStringBody(body, DataFormat.Json);

RestResponse response = await client.ExecuteAsync(request);
Console.WriteLine(response.Content);