For sites you wish to integrate which only give you an <iframe ... ></iframe>, you can try and use the src URL from the opening <iframe> HTML tag in an Embed() formula
E.g.:
<iframe width="x" height="y"
src="URL" // this is the URL you could try to use
title="Title" frameborder="z"
allow="something; something-else; ... ;"></iframe>
And in Coda, you could have something like:
Embed("URL_from_src")
If it doesn’t work (as embeds can be a little moody ), you could try :
Embed("URL_from_src", force: true)
Or, seeing they have an API and alike, maybe a pack could help too