TLDR:
add a markdown header: canonical_url: https://your.url.here

Took me some time to find this. The preamble is I'm using the basic markdown editor on dev.to. Or at least I was doing it until now. Somehow wasn't even aware of Rich+Markdown option arriving. Trying it for the first time right now - writing this little blurb.
Anyway - I needed to update some of my older posts with the canonical links that were missing.
Almost all the guides out there showed how to do it in the new editor - through the cog menu at the bottom of the editor.

But I needed to add it to older posts which were edited with basic.
I guessed it would come down to adding a header, but it took me a few googlings to find out which one.
So yes - just add this header:
canonical_url: https://your.url.here
And you're good.

Happy blogging!