You can now create messages using Heap CRM's webhook. Just place a title and message variable inside the JSON payload instead of the person/prospect data and a message will be created. Further, the message body will be scanned for the same commands as e-mail messages.
An example payload might look like something like this:
"title":"Talked to the Client",
"body":"Spent time with client \n\n[category:Meetings]\n[event:+1 Week:Follow-up with client]"