Pattern for REST Api wrapper

I have a third-party REST api where I should pass content as JSON. Parameters look like:

{ 
    id: id,
    fields:
    { 
        "TYPE_ID": "JOURNALIST",
        "SOURCE_ID": "CONFERENCE"
    },
    params: { "REGISTER_SONET_EVENT": "Y" }         
}

There are a lot of methods and count of parameters a variable. There could be only "id", or only "fields", also "fields" and "params" content could be varibale too.

I'm trying to make well designed wrapper for it using C#.

Now I'v created this classes to wrap request parameters and serialize to JSON:

public class BaseRequest
{
    public IEnumerable<RequestField> Parameters{ get; set; }
}

public class RequestField
{
    public string FieldName { get; set; }
    public object FieldValue { get; set; }
}

Where "FieldValue" could be a RequestField.

But I think it is not a good approach. Could you advise me any flexable and solid solution to use? Maybe any pattern for it or example of wrapper?