Run a Script when a customer adds a product to cart

Hello,
I am on Opencart 1.5.6.4 with journal template, and I want to add a tracker script to load when a customer add a product to cart. The script is this:

<script async src="https://examplescript.js" onload="FVPMSas.init(['example', 'cartpid', '201', '<?php echo $product_id;?>'])"></script>

Can anyone guide me?

Thank you