Two sited -- one PayPal account

I have two websites that use the same PayPal account. I asked PayPal how to set up the IPN to point back to the correct website after a customer completes a transaction and this is what they responded:

If you have multiple sites and each one uses a different IPN, you can call the individual IPN scripts by calling the IPN from your checkout rather than setting up the IPN URL in your PayPal account. To do this, you will need to pass the following line of code through your checkout:
<input type="hidden" name="notify_url" value="your IPN URL here">

My question is what zen cart page do i add this to?

Thanks,
Leslie

A quick search of the Zen Cart FAQs for your question brings up this article, which explains that there is no special extra setup required:
http://www.zen-cart.com/content.php?227-i-am-setting-up-two-zen-carts-and-would-like-to-use-the-same-paypal-account-for-both