I tried out the top answers and they did not work for me on Pop!_OS, but this did the trick:

https://stackoverflow.com/a/49426970/29685298

so (as linked response calls out change X_user and X_userid) ->

01 * * * * * sudo -u X_user DISPLAY=:0 DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/X_userid/bus notify-send 'Hello world!' 'This is an example notification.'