There are other methods as discussed in this thread[1] though, but using the BPIP extension is the simplest to use among others. The other method involves the modification of the URL and the use of epochconverter[2]  which is time consuming.

[1] https://bitcointalk.org/index.php?topic=5204486.0
[2] https://www.epochconverter.com/

It's still do-able with a minor work around which will take less than 20 seconds or so.

1) Navigate to the post you want to quote and click on its title. The page will reload with that post's message number in the URL (the last 8 digits). Copy these digits, but don't close this window yet.

2) In the thread you want to post the quoted message in, click on "quote" on any post. Once that reply page loads, paste your copied 8 digits over the 8 digits in the URL after "quote=". Hit enter to reload your reply page with the locked post quoted.

3) Go back to the window from step 1 containing the message you want to quote. Copy the 7 digits after "topic=". Go back to the window from step 2 containing your new reply, and paste these 7 digits over the 7 digits after "link=topic=" on the top line of your reply. This is to ensure the quote links back to the correct thread if someone clicks on it.