I'm creating a magic shield that grants the user an attack.  I'm selecting shield as the base item type due to the description that appears at the bottom - A shield is made from wood or metal and is carried in one hand. Wielding a shield increases your Armor Class by 2. You can benefit from only one shield at a time - that I cannot seem to be able to edit or remove.

I have added a Melee Weapon Attack modifier, but it doesn't show in my attack list.

Is this going to be a case of creating a custom attack?  Especially as it will be a bonus attack, not a regular attack.

Edit - Having played with the character generator, the battlerager class adds the bonus attack, not the spiked armor, so it looks like it's a case of customising the bonus attack or creating a custom attack.

I'd had a similar thought: a set of boots that offer two melee attacks - the first delivering a kick (bludgeoning damage), with the bonus attack coming via the spurs (piercing damage). Couldn't seem to get it working at all via the item itself, so I ended up creating two custom attacks for the character wearing them... But if there is a way of tying the two together, so these options are only available with the specific books equipped, I'd love to know!

It is not currently possible to add Actions to Magic Items.