Unfortunately, it's not possible to set a canonical link on Medium a posteriori.
Yeah, that's a problem. :/ I guess I'll continue rewriting my old content so I at least have a shot.
Furthermore, I don't know how Google handles cross-domain canonical reporting, but if I were writing a search engine, I would treat it as highly suspect.
Something like the following:
> Source: https://example.com
This should transfer link equity to your new blog post that helps offset any duplication penalty.
From the source: https://www.youtube.com/watch?v=mQZY7EmjbMA
I left Medium, but because I own all of the URLs no links broke. Nor are there duplicate copies with canonical links. So there’s no SEO risk.