@SoftwareBlacksmith I think the Chrome extension documentation's valid for the new Edge: https://developer.chrome.com/extensions. I was able to get a simple extension working with nothing but that documentation, published it to Google's Chrome Web Store*, then installed it on both Chrome and Edge.
(* Microsoft has yet to clarify whether there will be a Microsoft alternative to Chrome Web Store for the new Edge, although there's a rudimentary site, also linked to within Edge settings, that provides a few test extensions: https://microsoftedge.microsoft.com/insider-addons. A rep mentioned a while back on this forum that more information is coming.)
