This is not about systemd/systemctl and not about "onboot" so there's no rc.local trick here but it's the GUI/Gnome etc when a user logs in that a command is launched.
Put this in your home dir
The .config/autostart directory is where Linux Mint/Ubuntu checks for autostart application config files
mkdir -p ~/.config/autostart
Create a new entry/file that starts an application:
vi realtechtalk.com.desktop
The name can be anything but must have ".desktop" as the extension.
[Desktop Entry]
Type=Application
Exec=/usr/bin/firefox http://realtechtalk.com
Hidden=false
X-MATE-Autostart-enabled=true
Name[en_US]=firefox
Name=firefox
Comment[en_US]=Visit realtechtalk.com!
Comment=
The key point from above is the "Exec" variable and Name variable. Change exec to what you want to execute and change the name to something meaningful
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