The custom wondrous item builder only lets you add attribute, save or speed bonuses, plus resistances.
My specific use case is an official item from the Hoard of the Dragon Queen campain:
Insignia of Claws
Wondrous item, uncommon
The jewels in this insignia of the Cult of the Dragon flare with purple light when you enter combat, empowering your natural fists or natural weapons.
While wearing this insignia, you gain a +1 bonus to the attack rolls and the damage rolls you make with unarmed strikes and natural weapons. Such attacks are considered to be magical.
The custom wondrous item builder only lets you add attribute, save or speed bonuses, plus resistances. My specific use case is an official item from the Hoard of the Dragon Queen campain: