How to add custom header to ASP.NET Core Web API response

You can just hi-jack the HttpContext from the incoming Http Request and add your own custom headers to the Response object before calling return.

If you want your custom header to persist and be added in all API requests across multiple controllers, you should then consider making a Middleware component that does this for you and then add it in the Http Request Pipeline in Startup.cs

public IActionResult SendResponse()
{
    Response.Headers.Add("X-Total-Count", "20");

    return Ok();
}