How to use a php hook to pull data into PayPal from Simple Cart

I was wondering how to connect Simple Cart add-on to PayPal using the PHP hooks.

I’m new to using hooks, but would appreciate an example to get me going. I would like to display the ‘price’, ‘qty’ and ‘entry_id’ for an item and pass these to PayPal ‘Buy Now’ button form code, if that’s the correct way.

https://docs.reinos.nl/simple-cart/#php-api

<?php 
ee()->load->add_package_path(PATH_THIRD . 'simple_cart/');
ee()->load->library('simple_cart_api');
ee()->load->remove_package_path(PATH_THIRD . 'simple_cart/');

//save_cart_entry
ee()->simple_cart_api->add_cart_entry($entry_id = 0, $price = 0, $qty = 1, $tax = 0, $time = 1200, $extra_param = array(), $increment = true);
?>

<form action="https://www.paypal.com/au/cgi-bin/webscr" method="post" target="paypal">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="[email protected]">
<input type="hidden" name="currency_code" value="UKP">
<input type="hidden" name="item_name" value="">
<input type="hidden" name="item_number" value="">
<input type="hidden" name="add" value="1">
<input type="hidden" name="amount" value="">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="no_shipping" value="2">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="undefined_quantity" value="">
<input class="cart-btn cart-btn-secondary small_hide mr-2" type="submit" name="buy_now" id="buy_now_265" value="Buy Now">
</form>