Notice there is a “crosspost to” and a “crosspost from”. When users of Alexandrite or Lemmy clients initiate a crosspost, a link to the parent post is auto-generated and dumped in the body of the text tagged as “crosspost from”. We can edit that, and sometimes I do. AFAIK, there is no way to form that link to refer to the copy on the reader’s instance. The best we can do on that is to add the !<community>@<node> reference to the community, along with a title to search for.

The “crosspost to” line is not supplied or controlled by the author. It must be added by the reader’s server. In your case, lemmy.zip editorializes by adding that line when possible. The lemmy.ca and sh.itjust.works examples show that the url points to locally hosted posts. Only lemmy.ca can know about the lemmy.ca link. Lemmy servers are not sophisticated enough to find matching posts based on the body, and the title would not be reliable. So I believe the “crosspost to” line is only generated when a post makes use of the URL field. Since I referenced no URL in my post, there was nothing to compare for generating a “crosspost to” line.