Heyzine domain-restricted embedded code failing

I’m having trouble with
embedding some Heyzine content onto my site. I want to add some domain-restricted files onto my Wix site, but when I put in my link onto the “allowed” list, it still didn’t load correctly. I reached out to the Heyzine staff, and was told that “there are websites that internally embed within another domain that is not visible in the browser bar. To know exactly which one it is, you need technical knowledge and must “inspect the source code” to see the domain where it is embedded.” This is pretty far above my current know-how, and I’d really appreciate some clarity as to how to find that correct domain.

What I’m trying to do
Figure out exactly what link I’m supposed to add in to the allowed list so my content will load. My domain itself doesn’t seem to be it.

I tried checking based on the first results from Google but what I saw was from FlipHTML5 and while it didn’t work within the editor, it worked on the live site.

It did update work within the editor once I reloaded the builder.

I’m unsure if you have an exact link to it since I see there are a few sites that offer this tool.