Symptoms
When trying to use 'work_notes' as the field parameter of the sendMessage method in the Conversation API, the message is still posted as a comment.
Release
All
Cause
The Documentation is a bit incorrect and instead of passing two string parameters into the sendMessage method you should instead be passing an object.
Resolution
Call the sendMessage method with an object containing a body and field.
Here is an example that should work in a connect action.
var con = sn_connect.Conversation.get(conversation.sys_id);
con.sendMessage({body: 'testMessage', field: 'work_notes'});
Additional Information
Documentation of the method.