There are times when using the HTML element to embed codes would require JS script files in order to make sure the embed code populates on the live page.

Here's a short list of such apps and their script files along with the steps to locate the APP KEYs.

1 - Klaviyo - Klaviyo's embed code for the forms works just like any other embed code. However, if a form isn't populating on the live page, it is worth checking with the Klaviyo support to see if there is something wrong or if the script file needs to be added with the emebed code.
The script that Klaviyo uses can be found here.
<script type="text/javascript" async="" src="https://static.klaviyo.com/onsite/js/klaviyo.js?company_id=API_KEY"></script>
The API key needs to be added manually and can be found in the Klaviyo Account Settings.

2 - YotPo - If you are using the embed option from YotPo to display your review star ratings and the review widget, it is possible they may not populate on the live page. This may require the YotPo script to be added to the emebed code you are using.
You can learn more about the script here.
<script type="text/javascript"> (function e(){var e=document.createElement("script");e.type="text/javascript",e.async=true,e.src="//staticw2.yotpo.com/###YOUR APP-KEY###/widget.js";var t=document.getElementsByTagName("script")[0];t.parentNode.insertBefore(e,t)})(); </script>
The APP key needs to be added manually and can be found in the YotPo Settings.

3 - Stamped - Similar to YotPo, it is possible Stamped may require you to add their script file to the page. It is best to check with Stamped first.
You can learn more about the script here.
<script type="text/javascript" src="https://cdn1.stamped.io/files/widget.min.js"></script>
<script type="text/javascript">//<![CDATA[
StampedFn.init({ apiKey: '##publicAPIKey##', storeUrl: '##storeUrl##' });
// ]]></script>
The APP key can be found under API KEYS in the Stamped dashboard.

4 - Loox - Loox has a similar script that can be included to the page using the HTML element. You can even add the script code in the same HTML element that has the star rating or the review widget.
You can learn more about the script here.
<script async src="//loox.io/widget/loox.js?shop=STORE.myshopify.com"></script>
You'd need to manually include the myshopify URL in the script.