How do I convert a sitemap behind a paywall or login?

Public converters can only fetch publicly-accessible sitemaps — if /sitemap.xml requires authentication, the converter receives a 401 or 403 and can't proceed. Two fixes. First: download the sitemap manually in your authenticated browser session (just open the URL while logged in, then File → Save As → sitemap.xml), and use the Paste or Upload tab on xmlsitemapmaker.com. Second: if the site is yours, expose the sitemap publicly (sitemaps are meant to be public — Googlebot can't authenticate either). For competitor research, manual browser download is the only ethical path. Never put credentials into a public web tool, and confirm any data you upload doesn't contain sensitive URLs.