One of the problems of Microsoft's Edge browser not supporting Extensions right now is that password management with tools like LastPass can be quite difficult.

Here is a workaround with LastPass via Bookmarklets

You need the help of another browser - either IE or Chrome. 
Because Edge doesn't currently have a way to create/modify bookmarklets

1. In IE/Chrome.  Login to your LastPass https://lastpass.com/index.php?ac=1

2. Browser down Tools > Bookmarklet

 3. In IE/Chrome.  Drag LastPass Fill! Into the Favourites Bar (IE) or Bookmark Bar (Chrome)
 4. In Edge, go to Settings > Show the favorites bar > On
 5. Import from another browser - choose either IE or Chrome
 6. In Edge.  Once import is complete, drag the bookmarklet into the Favourite Bar.  I also deleted other favourites that I didn't need from the import.

7. You now have a Lastpass bookmarklet!

Summary

This works for Twitter and Reddit

I also hope as Favourites gets synchronized across Universal Windows Devices - this should work on Xbox One and Windows Mobile 10 as well.