Add custom element in Syndication Item or AtomEntry is impossible

[MTerance]

Unlike in the exemple RssWriteItemWithCustomElementExample.cs,
AtomEntry and SindicationItem inherit from ISyndicationItem.

when i want to add a custom element like a balise <enclosure> in my element <entry>,
after passing the formatter , my new elemnt is not displayed in the result :

                var item = new AtomEntry
                {
                    Title = post.Title,
                    Description = post.Content,
                    Id = $"{host}/posts/{post.Slug}",
                    Published = post.Published,
                    LastUpdated = post.Published,
                    ContentType = "html",
                };

item.AddContributor(new SyndicationPerson(post.Author.DisplayName, post.Author.Email));
                item.AddLink(new SyndicationLink(new Uri(item.Id)));

var content = new SyndicationContent(_formatter.Format(item));
content.AddField(new SyndicationContent("enclosure",  "linkToMyMp3.mp3"));

In the displayed result , the element enclosure created earlier is missing