In PureOS you can set an application to start on boot by using the GNOME Tweaks. To do it:
- Open GNOME Tweaks
- Select Startup Applications, and then press the + button
- Select the application you want to start on boot and press Add
And it is all done.
To remove an application from startup: go to GNOME Tweaks, select Startup Applications, and then press the Remove button to the right of the application that you no longer want to start on boot.