I am on a Mac running chrome as a browser. A client sometimes buys ads that show up on Microsoft Edge's start page. There's no URL where the page can be seen so I can't see even what it looks like. Is there a way, besides installing Windows 10 on a partition, to see their start page?
The Microsoft Edge start page is a selection of articles and ads "powered by MSN.com".
It appears to be an RSS feed in a "magazine layout" view, and in fact, some sources online report it is just the MSN.com news feed (http://www.thewindowsclub.com/msn-news-feed-edge-browser).
If the ads are included in the feed directly I would guess that he is actually advertising on MSN.com. However, if he is advertising JUST in the Edge start page, then I'd guess these are injected after leaving MSN control.
There does not appear to be a way to view this page or feed in its native state anyplace else, and so the solution to your issue may be to run Windows 10 in a virtual machine for this purpose.
Newer versions of Edge and IE can access a customized version of the MSN.com newsfeed. However, unlike other websites, the URL is not displayed on the browser's address bar. For some reason, Microsoft seems to prefer to keep these URLs a secret. However, the relevant URLs can be easily determined by launching the browser's Developers Tools (F12) feature. After pressing F12 click on the Network tab and refresh the page. The URL is in the first or second line under Name. Right click the line and click Copy URL. The hidden URL is now in the Windows clippboard.