As far as I can see you're asking for a macro group activation parameter that would watch for a given website to be active in your web browser.
Many people have asked for that feature, but it's not practical. Keyboard Maestro would have to constantly poll the browser (very inefficient) – and for that matter not all browsers have an API that allows for them to be polled.
If you're desperate for this feature there's a tricky method that might do the job, but you'll have to work for it...