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:
The name can be anything but must have ".desktop" as the extension.
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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