When you create an Incoming Web Hook, fields for both a message Title and Body are made available automatically, these map directly to the Title and Body fields in the outbox itself. Messages created in the Outbox are saved to your browser.
The Outbox can be put into a testing mode for web hooks by clicking the 'Test' button for the hook you'd like to test, in the Bips screen. Testing mode disables HTML formatting in the Outbox. See Testing Web Hooks for more info.