In my app I have Finder Sync Extension. I know how to launch it using pluginkit -a.
The problem is when I run from Xcode it launches and works. But I need this to be bundled with the main app and the main app should launch it. I used terminal command to achieve it, but it doesn't work.
I read few blogs and got to know, the plugin/extension/.appex needs to be registered to finder. How do I register it?
You don't need to launch a finder extension manually. The system will launch a finder extension for you. You just need to enable it in system preferences.
Have a look at App Extension Programming Guide for more details.