Advice on getting links to page sources for use on another page?

[itcarroll]

The download icon at the top of each page has a link to download the page source. I want to provide that same link somewhere within a parent page, and I can't figure it out!

Say the download icon resolves to https://example.com/prefix/_sources/a/b/child.ipynb.

What I've tried in the parent (Markdown) is:

View the [page] or download the [source].

[page]: b/child
[source]: ../../_sources/a/b/child.ipynb

I get a WARNING: 'myst' cross-reference target not found and [source] ends up resolving to https://example.com/prefix/a/parent.html#../../_sources/a/b/child.ipynb.

Is there a way to start an "external link" poiting at the root of the site? Is there a role that returns the sources of a page?