The default enchantments edit menu can be used to configure the default enchantments of a custom item. If the item has a single default unbreaking enchantment of level 2, it should look like this:

On the left are the 'Cancel', 'New Enchantment' and 'Apply' button.

Additionally, there is a red cross, enchantment button ('Unbreaking' in the screenshot above) and an edit field for the enchantment level ('2' in the screenshot above).