Magento 1.8 Cart – Fix Adding to Cart with Querystring Redirecting to Homepage

The link you're describing is created using the users local session. As far as I know it's impossible to access this through the SOAP API.

Instead use the Checkout SOAP API to add a product to cart. Documentation on that can be found here.

This does mean however that you will need to place some of your own code in between that shows the user an add to cart button and calls the API when the users clicks it

First way:

1. You'll have to override app/code/core/Mage/Catalog/Block/Product/List.php block and create a new function which will load your custom form which use to insert birth day etc.
2. Then in the template file app/design/frontend/base/default/template/catalog/product/list.phtml around line 61 and line 103 invoke your custom function instead of getAddToCartUrl($_product) function in the onclick event of Add to Cart button
3. Make sure to invoke getAddToCartUrl($_product) function inside your custom function after submitting your custom form.

Second way:

1. Write an event/observer to inject your custom form when user click the Add to Cart button. You can try with this event 'checkout_cart_save_before'.

Note: I didn't test this event. If it didn't work refer here for achieve the relevant one http://www.nicksays.co.uk/magento-events-cheat-sheet-1-7/