Hi,
The library supports both versions. The difference is how the client is instantiated. To use the beta API in .NET, create the client like this:
pdfcrowd.HtmlToPdfClient client = new pdfcrowd.HtmlToPdfClient(username, apikey);
More examples here:
https://pdfcrowd.com/api/html-to-pdf-dotnet/#examples-url-to-pdf