Eiher highlight something in their post as tiger mentioned or reply and click the “quote whole post” button in top left of editor (preserves formatting)
You can also press ‘?’ on your keyboard to open up a list of keyboard shortcuts. I only know this because I read about it on another Discourse forum, and I’m not sure how they found out. It says you can press ‘q’ to quote but that doesn’t seem to be working for me.