Works fine
$ curl -k --location --request POST 'https://192.168.122.11/reboot' --form data='{"op": "reboot", "path": ["now"]}' --form key='foo' {"success": true, "data": "Warning: there are unsaved configuration changes!\nRun 'save' command if you do not want to lose those changes after reboot/shutdown.\n\n", "error": null}