Open a terminal and do:
leafpad /home/user/.config/reminder.sh
replace user with your username.
add the following to the file:
#!/bin/bash
sleep 3
echo "This is my reminder" | xargs -0 notify-send
save and close the file.
Then in the same terminal do:
chmod +x /home/user/.config/reminder.sh
replace user with your username.
Then do Menu, Settings, Session and Startup, Application Autostart tab.
Click on +Add in Name and Description put My Reminder in Command put:
sh /home/user/.config/reminder.sh
click Ok, then Close.