It still works for me. I don't think this has ever been officially supported, but it definitely works for me. I can't guarantee it will work on all pods (I'm on app-sjf, for what it's worth).
Below is a screenshot of my webhook.I created a custom field "Marketo Cookie ID" in SFDC. It's up to you to populate that with the correct Cookie ID. My Smart Campaign is as follows:
- Smart List
- Trigger: data value changes, field is "Marketo Cookie ID", new value is not blank
- Trigger: lead is created, source = salesforce.com
- Marketo Cookie ID is not empty
- Call Webhook "Associate Cookie ID"