How to download macOS High Sierra from the Mojave App Store?
In previous versions of the beta this was not possible.  But in the latest release (as of 9/15/18) it is possible to use direct links to open the download page.

This links can be found on Apple's How To Download pages.  

Here are the direct links for a some of them:


High Sierra
Sierra
El Capitan
Yosemite


All other OS can be purchased from Apple directly and are not in the App Store.