Also understand that “add on the fly” is not so much a specific feature–there are no buttons to click or different screens that pop up–as a benefit of being able to open two browser windows at the same time. You are actually viewing two instances of the database, if you will. One is entering the main transaction, such as a purchase invoice. The other is looking at a different tab, say Inventory Items, and possibly creating new entries there.
So if you adopt the server or cloud editions, don’t be disappointed when you can’t find the “add on the fly” button.