Sellfy Buy Now Button Code Question

Site URL: https://docs.sellfy.com/article/132-how-to-customize-buy-now-buttons-for-my-website#customize_embed_code_to_have_custom_button

Sellfy provides code for a “Buy Now” button.  There are also instructions that you can use to have an image act as your Buy Now button.  That is what I am trying to do.  See below for Sellfy instructions.  

I uploaded my image to My Custom Files.  I added my Product Key and my image URL to their code and put that new code in a code block.  It doesn’t work.  Is that CSS below Squarespace-compatible?  Any suggestions?  Thank you for any and all responses.

From Sellfy:

Custom button or image
Here's an example of how your button and code may look like with custom class. In the code below, the link to the button image is https://example.com/images/button.png, which is just an illustration of what the image link can be. 

You can generate such a link for your custom buttons by uploading the button image onto a server. You may also use a reliable online button generator that will produce such a link for you.

<a href="https://sellfy.com/p/[PRODUCT-KEY]/" id="[PRODUCT-KEY]" class="sellfy-buy-button sellfy-buy-button-custom"><img src="https://example.com/images/button.png"></a>
<script src="https://sellfy.com/js/api_buttons.js"></script>

Cut and pasted from:

https://docs.sellfy.com/article/132-how-to-customize-buy-now-buttons-for-my-website#customize_embed_code_to_have_custom_button