After installing now you can start for the basic setup.

Go to Plugins section and make a search for PWA
link: administrator/index.php?option=com_plugins

You will see the Plugin with title : " System - Push Notifications"

Click on to the title and you reach to back-end of the plugin for setup.

ONESIGNAL SETUP

  1. First of all, you  must create an account at Onesignal.
    Onesignal is the market leading self-serve customer engagement solution for Push Notifications.
    There are several free features that you can use easily.
  2. Visit https://app.onesignal.com/signup
  3. After signup, choose free plan and Check Your Email to activate your account.
  4. After replying the on-boarding question you can create your first app.
    Give a name to your app, e.g. My Website App
  5. Click Complete Later at Start Guide
  6. Click to Setup Platform
  7. Choose Web Push Platform - All Browsers
  8. Click to Custom Code at opening page
    IMPORTANT : IF YOU DO NOT CHOOSE CUSTOM CODE, YOU MAY HAVE PROBLEMS, IT IS NECESSARY WHEN YOU ARE USING PWA
  9. Scroll down and Make the Site Setup 
    Write your site name 
    Enter your site URL
    Upload a 192pixel * 192 pixel Icon
  10. Click to SAVE

That is it,

You only need App ID, copy App ID to submit in plugins backend Plugins > System - Push Notifications > Start > OneSignal App ID

If you have difficulties to find App ID, click here to read where to find it.