Microsoft Edge is a new browser and it's not a standard Windows Application, but a 'Modern App'. Modern Apps are sandboxed and have limited access to your computer resources (for security reasons) .
This means that other applications (like WWB) cannot launch it the usual way.
We have implemented a workaround to bypass this Windows limitation in version 10.4.3 (and newer).
To add Microsoft Edge to the preview browser list open 'Edit Browser List' and click 'Auto detect'. The browser should be automatically detected by WWB and added to the list.
Note that Microsoft can change this functionality at any time, so we may have to update this in future updates again.
On some Windows 10 computers, previewing in Microsoft Edge may not display images and other assets during preview.
In that case go to Tools->Options->Publish & Preview and uncheck 'Disable IE security warning during preview'.