Best answer by TripleHelix
Here it is on the Microsoft Store: https://www.microsoft.com/en-us/p/webroot-web-threat-shield/9nt6c2z1hfnm?SilentAuth=1&wa=wsignin1.0&activetab=pivot:overviewtab
Perhaps that's why is so difficult to find this Webroot extension for Edge. It doesn't show in the Edge list of available extensions and I couldn't find a direct link for installing it. It pops up randomly in the Edge browser for a few seconds - if you miss that notification there doesn't seem to be any obvious way to install it.