CreatePostUpdated.cs

[HttpPost(ApiRoutes.Posts.Create)]
public Task<IActionResult> Create([FromBody] CreatePostRequest postRequest)
{
    var post = new Post {Id = postRequest.Id}; // <-- this is called mapping
    
    if (string.IsNullOrEmpty(post.Id)
      post.Id = Guid.NewGuid().ToString();

    _posts.AddPost(post);

    var baseUrl = $"{HttpContext.Request.Scheme}://{HttpContext.Request.Host.ToUriComponent()}";
    var locationUri = baseUrl + "/" + ApiRoutes.Posts.Get.Replace("{postId}", post.Id.ToString());

    return Created(locationUri, post);
}