I'm a beginner with websites and I want to know if the sitemap.xml file of my website should or shouldn't index the 404.html page, which is a custom 404 page.

This question is quite similar to Should 404 pages feature in your sitemap.xml but that question felt very vague to me and it didn't clearly state what I'm supposed to do if I have a custom 404 page. Though there was a short comment about it, yet I need to verify that.