Modern desktop environments like GNOME offer UI for this, but if you’re using a more bare-bones window manager, you’re on your own. This article outlines how to get a login page opened in your browser when you’re behind a portal.

If your distribution does not automatically enable it (Fedora does, Debian doesn’t), you’ll first need to enable connectivity checking in NetworkManager:

# sudo apt install network-manager-config-connectivity-debian

Then, add a dispatcher hook which will open a browser when NetworkManager detects you’re behind a portal. Note that the username must be hard-coded because the hook runs as root, so this hook will not work as-is on multi-user systems. The URL I’m using is an always-http URL, also used by Android (I expect it to be somewhat stable). Portals will redirect you to their login page when you open this URL.

# cat > /etc/NetworkManager/dispatcher.d/99portal <<EOT
#!/bin/bash

[ "$CONNECTIVITY_STATE" = "PORTAL" ] || exit 0

USER=michael
USERHOME=$(eval echo "~$USER")
export XAUTHORITY="$USERHOME/.Xauthority"
export DISPLAY=":0"
su $USER -c "x-www-browser http://www.gstatic.com/generate_204"
EOT