update-product-description-with-ipay88-for-give.php

<?php
function mg_update_product_description( $productDesc, $donationId ) {

	// Do whatever you want to add using the Form Id or $_POST and update the product description and return it.
	$purpose = give_get_meta( $donationId, 'purpose', true ); // `purpose` here is the name of the FFM field.

	return $purpose;
}

add_filter( 'mggi88_update_product_description_text', 'mg_update_product_description', 10, 2 );