How can I add a custom code handler?

You can use the 
addCodeHandler() method to add custom behaviour for specific codes when applied to the cart.
For example you can create a discount code that adds a free sample pack to the cart.
First, define some settings to control this behaviour:
\Aero\Common\Facades\Settings::group('free-sample', function (\Aero\Common\Settings\SettingGroup $group) {
    $group->eloquent('discount', Discount::class);
    $group->eloquent('variant', Variant::class)->searchField('sku', 'sku');
});
Then add a code handler to the cart that consumes these settings:
\Aero\Cart\Cart::addCodeHandler(function (Cart $cart, string $code) {
    $discount = setting('free-sample.discount');
    if (! $discount) {
        return false;
    }
    if ($code !== $discount->code) {
        return false;
    }
    $alreadyApplied = $cart->items()->contains(function (\Aero\Cart\CartItem $item) {
        return $item->meta->get('is_free_sample');
    });
    if ($alreadyApplied) {
        throw new CartModificationException(__('Discount code already applied.'));
    }
    $cartItem = new CartItem(
        setting('free-sample.variant'),
    );
    $cartItem->setName('Free Sample Pack')
        ->setImmutable()
        ->setNotCombinable()
        ->setFree()
        ->setMeta('is_free_sample', true);
    $cart->add($cartItem);
    return true;
});
The 
aerocargo/automated-add-to-cart module can be used to automatically add items to the cart based on rules.