Assuming the script code looks something like this:
<div id="ABC"></div><script src="https://www.barattalo.it/adserve.php?t=ABC&f=4">/*300x250*/</script>
there’s a good chance that <div id="ABC"></div> is the target that the script at <script src="https://www.barattalo.it/adserve.php?t=ABC&f=4"></script> injects the ad into.
I’m guessing a bit here, but it’s likely that the exact same script tag is generated for all ads served from your adserver. If that’s the case, the approach outlined here might work for you: Is there a plugin for revive-adserver for discourse? - #3 by simon.
To test it out, create a theme component from the the Admin / Customize / Themes section of your site and add the script you’re seeing in the embed code to the head section of the theme component. Then include that theme component on all you site’s themes. After having done that, just add the embed code’s target div element to your house ads. For example <div id="ABC">.
If this doesn’t work, check your web inspector’s console for any errors. One thing to note is that there can be some unexpected issues with disabled ad blockers: Chrome adblock extension house ads issue.