From the docs:
On the desktop and mobile apps, an alarm can be associated with any to-do.
How?
select a note, then select "switch between note and todo type" then set an alarm (with the watch on desktop toolbar)
let me know if it works for you as i have some problem with the alarm 
Ah, I needed to make the window much wider to see the alarm clock.
Yet another reason for a single panel mode.
And yes, the alarm works: I get a notification showing the markdown text of the complete note. But only if Joplin is actually running.
AMdroid on a phone is about the best alarm app I've found, but that precludes the desktop of course - unless you have PhoneLink running.
Finding 'alarm' wasn't the most straightforward process to begin with. Search here on desktop browser in the FAQs did find it in ToDos, but for mobile it gave half an answer:
On mobile: Open the note actions menu, then click "set alarm".
..assuming we know what an actions menu is. [three vertical dots btw].
Like most things in Joppo you have to scratch your head a lot to figure out what controls are for: search for 'insert code' {;} and you get a programmer's dissertation on 'style something', and similarly for 'insert HTML tag' </>. A case of 'if you don't know, you don't need to know' perhaps.
I'd like to surround text with and that didn't exactly work - but that's a story for elsewhere.
Examples would be fantastic.
Any preview here before I reply...?