Sometimes the treatment you'd like to recommend is something you need to simply describe (a behavior pattern to encourage, or a general recommendation). You can still think of it as a "treatment" even if it's just text. So click on the "Add Treatment" button under the "Assessment" editor, and then just start typing. As soon as the system realizes that the characters you've typed do not match anything in your inventory, it will stop trying to suggest auto-complete options. Anything you type there will be kept as part of the record, but will not be added to the invoice.
Here's an example:
