I was traveling and trying to connect my Omarchy laptop to the hotel Wi-Fi. It had a captive portal, but the login page wouldn't show up.

What worked was opening a plain HTTP page:

xdg-open http://captive.apple.com

There's nothing special about captive.apple.com. The important bit is using http:// rather than HTTPS. That gives the Wi-Fi network a chance to redirect the request to its captive portal.

What is xdg-open?

xdg-open opens whatever you give it using the default application for it. Give it a URL, and it opens your default browser. Give it a PDF, and it opens your PDF viewer.

It's basically the Linux equivalent of open on macOS.