how to change add to cart link per product?

-- HikaShop version -- : 2.4
-- Joomla version -- : 3

hi,
How can I change link Add to cart for each product ?
for example:
product 1: click on add to cart button -->go--> www.site.com/1.html (this link external)
product 2: click on add to cart button -->go--> www.site.com/2.html (this link external)

please help me
thnx

or
how add php code to show link for per products
i use this code but dont work

<?php if($this->params->get('product_id') == 1) { ?>
<a href="http://mywebsite.com/link/to/all/products/page/for/module/with/id/XXX">all projducts</a>
<?php } ?>