On a Mac, I highlight the words I want to quote, whether it's a sentence or the entire post, then hit "Apple + C" (the key with the Apple symbol + the C key). Then when I open the "reply to thread" box and have a cursor blinking, I hit "Apple + V" and the highlighted words reappear there. I then add the quote marks manually.
Probably a kludge way of doing it, but what do I know?