You aren’t missing anything, I’m afraid. That switch is part of the forum software rather than Agenda, so the side by side raw and formatted view you get when writing a post here is not something the app has.
Agenda isn’t a markdown editor as such, but it does understand quite a lot of markdown as you type. Start a line with # for a heading, - for a bullet, - [ ] for a checkbox, backticks for code, and so on. So the quick typing part of what you want already works, it just formats as you go rather than showing you the source alongside it. There is a full list here: Markdown Cheat Sheet
Going the other way, Edit > Copy As Markdown gives you the markdown source of a note.
A proper markdown mode with the source visible is a fair request though, and it isn’t something we have looked at. I’ll take it along.
Kind regards,
Drew