Find the post you want to quote, and click on its title to reload the page to that specific post. Then copy the message number (last 8 digits) from the URL.

Then, in the thread you want to reply in, click on "quote" on any post. In the URL bar on the new page, paste over the 8 digits after "quote=" with your new 8 digits. Press enter.

To ensure the quote links properly, you then need to take the 7 digits after "topic=" from the original post (as in step 1), and paste them over the digits after "link=topic=" in the body of your reply.

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Edit: reformatted and added in final step. Thanks to shahzadafzal and my dream2021.