You need to enter the webinar ID. In your site’s logs in /logs, you should be able to see more details if it still can’t add the event in the modal window. I also fixed the URL issue you mentioned, thanks for reporting that.
Unfortunately, no, this currently does not provide a vendor-agnostic workflow. The Zoom API calls are abstracted though, so it’s possible to add support for another vendor with a bit of effort (although, the plugin would need to be renamed to something less zoom-y).