It depends on what you want to quote.
1. If you are reading a Private Message from another forum member, you reply by clicking the
button and it automatically quotes the message that the other user sent you.
2. If you want to send a Private Message to another forum member and quote a post they made in a forum thread, there isn't a forum button for that, but you can get the same effect.
Suppose I want to quote the first post in this thread while sending Laura a Private Message about it:
a. I click the Quote button under that post.
b. Instead of typing anything in the message box, I select all of the text and pick Edit -> Copy in my web browser.
c. I click the Back button of my web browser since I didn't really want to post anything.
d. I start a Private Message to Laura by clicking her name, picking "Send a private message to Laura", and typing a message title.
e. I click in the message area and pick Edit -> Paste in my web browser.
f. I type my message below that.