Refer to below example –
I have a RouteConfig class:
public class RouteConfig
{
private static string ControllerAction = "ApiControllerAction";

public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

routes.MapHttpRoute(
name: ControllerAction,
routeTemplate: "api/{controller}/{action}/{id}",
defaults: new { id = RouteParameter.Optional }
);
}
}

The call into the API uses JSON.Stringify:
$.ajax("http://localhost:50762/api/values/PostSavePage/", {
data: JSON.stringify(jObject),
contentType: 'application/json',
type: 'POST'
});