Hi,
The easiest is that now that you have a product with the custom item field setup, you could create a menu item of either the type "products listing" or "product page" and select in the settings of the menu item either the category of the product if it's a listing or the product itself if it's a product page, and you'll be able to access the product page of the donation product with the donation field and the add to cart button so that people can use it to do a donation.
If you want to integrate the add to cart and field in a Joomla article, you won't be able to use the content tag system of HikaShop as it doesn't support custom fields display as Mohamed pointed out. Instead, I would recommend to configure a HikaShop content module via the Joomla modules manager in order to display your product as a listing of products and then include that module in your Joomla article with the Joomla loadposition system:
docs.joomla.org/How_do_you_put_a_module_inside_an_article%3F
That would allow you to achieve pretty much what you want without custom code.