By default, quoting replies will get posted to the same thread as the quoted reply. There is a way in which one can post it in a different thread,  These instructions are for someone using a computer. I don't know if it's possible with other devices.
Draft your reply including the quote as normal. But don't post it. Instead click on the source code icon at the right end of the icons in the add reply screen (the <> one). That takes you to a different box headed Source code. Highlight the contents of that box. Press Ctrl-C to copy the text. Click OK or cancel to come out of the Source code box back to the Add Reply box. You don't want to post the reply and you want to abandon it by clicking on say the Big Brother UK link at the top.
Now go to the thread that you want to post your reply in. Go to add a reply but don;t type anything there, Click on the source code icon. In the box headed Source code press Ctrl-V to paste the text that you had copied into the box. Then click OK and the reply that you had drafted should be there for you to post.