By declaring the jsonString parameter with [FromBody] you tell ASP.NET Core to use the input formatter to bind the provided JSON (or XML) to a model. So your test should work, if you provide a simple model class
public class MyModel
{
public string Key {get; set;}
}
[Route("Edit/Test")]
[HttpPost]
public void Test(int id, [FromBody] MyModel model)
{
... model.Key....
}
and a sent JSON like
{
key: "value"
}
Of course you can skip the model binding and retrieve the provided data directly by accessing HttpContext.Request in the controller. The HttpContext.Request.Body property gives you the content stream or you can access the form data via HttpContext.Request.Forms.
I personally prefer the model binding because of the type safety.