How to add raw text to a c# RestRequest

Question

I am working with an API which I need to filter the POST by a start and end date. I can get this working in Postman which is 'raw text' but I cannot determine how to move this across to my C# asp.net RestRequest. The Postman code is here returning the correct data:

And this is what I'm trying in c# but it is being ignored and just returns all the rows. So I can get data from the API but cannot add the request body so it doesn't ignore it.