Create OS X Widget Today Without Host App
You cannot do this simply because your executable is still running in the companion application and not in the extension. This means that the codes are actually owned by the companion. So, you should have your companion application.
Information from the official documentation:
The app extension is different from the app. While you must use an application to contain and deliver your extensions, each extension is a separate binary that runs independently of the application used to deliver it.
source to share