How can I scan a barcode?

The app features a small scan icon that can be found right next to the barcode entry field. You can tap that or, at the top of the overlay, you can tap "< Add item", which will both take you to the same page.

The app remembers, what you have last used, so when you have used the barcode scanner to add an item the last time out, it will take you there. Instead, if you have manually added an item last time around, you will be taken straight to the item details page.