Create a new item named Defend , Then go to Abilities and add defend to the item . You find this line in object editor in the top lines ;D . I don't remember that defend is passive or not . You can just make turn on , and remove turn off . For example when a unit carries this item , make unit use item directly by triggers and then it can't be off ;D since you have disabled the ID from object editor .
Wish you understood ._.