Right-click on the post # which is on the far upper right of that post. Select "Copy link address" and then you can paste that into your table of contents. You may also be able to use the "share" icon and copy the link; that doesn't work on the first post but should on subsequent posts. Use the "Insert link" button if you want to show words rather than simply the link. The link will include "/post-" and a string of numbers at the end.
Both of these will link back to your original post: