magento-1.9 – Magento Community Form Key from External Site URL

I have an requirement to allow products to add to the cart from an external link, like in an email campaign. I've created an observer to allow a form key to be added to the url if it doesn't exist, however it only adds if the session already exists (if I visit the site then use the URL it works) otherwise I get a cms no cookies url. Am I using the correct event to hook into for pre-cart but post-session start?

The URL I'm tyring to get working is:

/checkout/cart/add?product=1&qty=1

config.ml:

<?xml version="1.0"?>
<config>
<modules>
    <Vendor_GenerateFormKey>
        <version>1.0.0</version>
    </Vendor_GenerateFormKey>
</modules>
<frontend>
    <events>
        <controller_action_predispatch_checkout_cart_add>
            <observers>
                <Vendor_GenerateFormKey_Model_Observer>
                    <type>singleton</type>
                    <class>Vendor_GenerateFormKey_Model_Observer</class>
                    <method>GenerateFormKey</method>
                </Vendor_GenerateFormKey_Model_Observer>
            </observers>
        </controller_action_predispatch_checkout_cart_add>
    </events>
</frontend>
</config>

Observer.php

<?php

class Vendor_GenerateFormKey_Model_Observer
{
//Put any event as per your requirement
public function GenerateFormKey() {

    $oldkey = Mage::app()->getRequest()->getParam('form_key');

    $domain = Mage::getBaseUrl();

    if (empty($oldkey) && ($domain == 'http://site1.co.uk/' || $domain = 'http:///site2.co.uk/')) {

        Mage::getSingleton("core/session", array("name" => "frontend"));

        $product = Mage::app()->getRequest()->getParam('product');
        $qty = Mage::app()->getRequest()->getParam('qty');

        $key = Mage::getSingleton('core/session')->getFormKey();

        if (empty($product)) { //if no product id, redirect to homepage
            Mage::app()->_redirect('');
        } else { //redirect to the add to cart with the form key

            $url = 'add/product/' . $product . '/qty/' . $qty . '/form_key/' . $key;

            Mage::app()->getRequest()->setParam('return_url', $url);
        }

    }

}
}

I had a look at several 'how tos' for this but I can't seem to find a solution that wants to work with Magento 1.9.2.3. I know the form key doesn't last past the user session, hence the observer to add the form key only once the user is on the site.