According to the Arch Wiki you need to place the .desktop file in:
~/.config/autostart/*****.desktop
You also need to add the following line to the .desktop file:
X-GNOME-Autostart-enabled=true
But that’s probably the hard way to do it, I think the settings panel should also have a Startup Application tab.
surprised_pikachu_face.jpeg
Check your journald and/or make your script log it’s actions.