What is the ASP.NET Core equivalent to HttpRequestMessage?
In .Net Core they have merged the Web API and MVC so you could just do it like this with IActionResult or one of its derivatives.
public IActionResult Post([FromBody]JToken jsonbody){ // Process the jsonbody return Created("", null);// pass the url and the object if you want to return them back or you could just leave the url empty and pass a null object}