How to properly setup a modal shop with FastSpring?

I am currently exploring FastSpring, but their examples are very poorly documented so I struggle to understand even the most basic examples on how to create a modal dialog for my products. In the link below, they have an example for a "Buy The Photo.io App" but their respective JavaScript code fails on my test webpage.

https://fastspringexamples.com/full-cart/modal-carts/

In specific, in line #176:

var fsb_modal = document.getElementById('fsb-modal');

After copying parts over to my test webpage, this is where the script breaks. Were does fsb-modal come from? Where is this created? There is no explanation how the "JavaScript" code is embedded into the HTML. Any help is highly appreciated!

P.S. My test web-page includes the script directive from my test-account:

<script
    id="fsc-api"
    src="https://1234567890.cloudfront.net/sbl/0.8.3/fastspring-builder.min.js"
    type="text/javascript"
    data-storefront="x-y-z.test.onfastspring.com/popup-x-y-z">
</script>

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