How to add Markdown to a page via REST API?

[Jeremiah Watkins]

I'm working on a method to programmatically post Markdown to a Confluence page via the REST API. With the REST API documentation, I've figured out how to create/update pages. The problem is, the only way I know how to add Markdown/Confluence wiki to a page is by manually hitting the "Insert Content" button and selecting "Markdown". So how would I do this via the REST API?

I found this question from 6 years ago that doesn't help, and both Jira issues have been open without update for almost as long.

Any ideas?