Like @DBear mentioned, one of Hype's primary use cases is in producing web ads. Hype Pro even has a plugin system to streamline a lot of the export workflows for add systems called Export Scripts.
It probably would be a better idea to get more specifics from your clients on the "standalone" aspect. If they just mean no external network requests, then this is the typical Hype output unless you've done something to add network request.
If they mean a single file, that's a different story. Web formats typically do not package things into a single file; usually there's a folder of assets with an initial index.html file. Most advertisers accept a zip file with all the assets.
That said, some do want a single .html file with all assets embedded and no external requests. This is a little bit insane for a myriad of reasons and goes against the grain of the web and advances in web server technology, but that's a rant for another day...
Hype typically does export a .html file and a folder of resources. You can learn more about the standard export on this documentation page.
So it is not typical that Hype outputs a single file. But, in a case where your ad has no bitmap images, this is not too hard to do with Hype. In the Advanced Export, there's an option to "inline data+loader" file which can put a lot of contents into the .html file. You'd also need to use the CDN option for the runtime, or paste the runtime file into a script tag after export. You can potentially use Hype's vector shape tool for simple graphics, or embed SVG content as the inner html of elements.
However if you have other image assets, those would need to be inlined via more challenging means. Typically you'd need to encode images as base64 strings. There's discussion and tips/tricks on the forums on how to do this, like this tool, but the reality is it isn't entirely streamlined and there are many caveats. Due to base64 encoding being larger than the actual files, you are also likely to blow past ad file size requirements quite quickly.