In this example I will autostart the GUI package manager of Manjaro.
Create a file e.g. ~/.config/autostart/pamac-manager.desktop
In that file put:
[Desktop Entry]
Terminal=false
Name=pamac-manager
Type=Application
Exec=/usr/bin/pamac-manager
Icon=pamac-manager
Comment=Package Manager
In the Exec
line put the path to your executable file. The rest you can probably guess.
Source: https://specifications.freedesktop.org/autostart-spec/autostart-spec-latest.html