A "destination" is a WROLPi instance that the extension can send to. Configure one or more here, mark one as default, and pick which to use per-action from the popup.
Examples:
https://wrolpi.local ·
https://10.0.0.5:8443 (Docker) ·
https://10.0.0.9 (Raspberry Pi / Debian)
Or open /admin/extension on your WROLPi to copy the exact URL.
Firefox users: after clicking Test, also click the extension icon in the toolbar → Always Allow on (your WROLPi site). Firefox needs both grants — the Test button covers API access; the toolbar grant lets the extension mark each WROLPi tab as connected. (Chrome, Brave, and Edge handle it in one step.)