That sounds like an interesting challenge...
We begin by looking at the posting history of
@PeterDonis and locate a recent thread. There it is right at the top. Something about being closed for moderation.
We scroll back in that thread to find the comment in question. There it is.
We click the little "share this link" on the post to see what the URL looks like:
https://www.physicsforums.com/threads/general-relativity-question-gravity-clock.1066746/post-7130447
We quote someone else's post to see what the BBcode for a quote looks like. For instance:
[QUOTE="PhDeezNutz, post: 7130482, member: 577620"]
So all we seem to need is a post number (7130447) and a member number.
Looking at the profile for
@PeterDonis and extracting the URL we find:
https://www.physicsforums.com/members/peterdonis.197831/
So the requisite member number seems to be 197831.
Let us construct BBcode for the article in question:
[QUOTE="PeterDonis, post: 7130447, member: 197831"]
And test it:
Huzzah! It works.