How to add a deadline to an event or note?
We don’t really have deadlines. You can set the date or range of dates on a note. This is the dates it is relevant. Just select the note, and tap the calendar button top-right on the note.
For deadlines, you are probably best to set a Reminder. Use the Insert > Reminder option or type in \remind
You can set a time/date for that, and your Reminders app will notify you.
Lastly, you can add a #due(25/12/2021) like tag in your note. You can then search for that tag, and save the search as a Due overview. That overview will show any notes that are past the due date.