Here’s how to do it:
To link to a post you click the icon at its lower right that looks like two links of chain on the diagonal. It’s just to the left of the “…” icon.
When you click that icon, a box will pop up with a highlighted url that points to that post and gives you credit for it.
Click “Copy” in your browser however you do that. Then paste that into your other post in the other thread.
Here’s what it looks like: I went to the current last post in this other ATMB thread and put the link I got from the chain icon there into here:
When you go look at that post by following the link in this post, you’ll see that at the bottom of that post in gray there will be a link back to this post. Labeled with the title of this thread. That back-link is what we’re talking about. It was added to that thread automatically as a result of my post in this thread.