C# strings use double quotes. Try ${event.orderby || "description desc"} and make sure the property casing is correct e.g. the property is called description and not Description.
Not sure what in the post I linked is like your initial post. This code ${event.orderby || 'description desc'} is not C#, it's a JavaScript expression used in Angular Radzen application. The post I linked shows how to set $orderby parameter for data source method invoke and again to restate the options:
you can use column SortOrder property to define the initial column sort order
you can sort your data using the $orderby parameter