Not sure exactly what you’re asking here. But quite possibly, the answer is: It happens automatically. Are you referring to Post #9 in that thread by madsircool which has, just following it, a link to my quote of it in this thread?
If you’re asking how that link got there, it happens automatically. When you post a link on one post to another post, Discourse automatically adds that cross-reference link at the bottom of the linked-to post. At least, I think that’s what you’re asking. (It may only happen depending on just how you create the link to another post. (?))