How to use Created (or CreatedAtAction / CreatedAtRoute) in an asp net core api

CreatedAtAction gives the best output in my opinion. The following controller code will do what you need:

[Route("api/products")]
[ApiController]
public class ProductsController : ControllerBase
{
    private readonly IProductRepository productRepository;

    public ProductsController(IProductRepository productRepository)
    {
        this.productRepository = productRepository;
    }

    [HttpPost]
    [Route("")]
    [ProducesResponseType(StatusCodes.Status201Created)]
    public ActionResult<Product> CreateProduct(ProductCreateDto product)
    {
        if (product is null)
            return BadRequest(new ArgumentNullException());

        var entity = productRepository.CreateProduct(product);

        return CreatedAtAction(nameof(GetProduct), new { id = entity.ID }, entity);
    }

    [HttpGet]
    [Route("{id}")]
    public ActionResult<Product> GetProduct(int id)
    {
        return productRepository.GetProduct(id);
    }
}

Issuing the following request:

POST http://localhost:5000/api/products HTTP/1.1 
Host: localhost:5000
Connection: keep-alive 
Content-Length: 25 
Content-Type: application/json

{ "name": "ACME Widget" }

Will yield the following response:

HTTP/1.1 201 Created
Date: Mon, 12 Oct 2020 09:50:00 GMT
Content-Type: application/json; charset=utf-8
Server: Kestrel
Content-Length: 29
Location: http://localhost:5000/api/products/1

{"id":1,"name":"ACME Widget"}