Your webhook indeed needs to:
* Be accessible from the internet
* Be reachable over HTTPS
* Correctly respond to a "challenge" request
see https://www.adobe.io/apis/experienceplatform/events/docs.html#!adobedocs/adobeio-events/master/intro...
So you cannot use slack webook url straight from the Adobe developer console
You will need a webhook to proxy this to slack,
Note you could use a simple Adobe I/O Runtime action for this or any other serverless function