I suggest to update angular to at least Angular version 7.1.0 ( I would suggest to maintain up to date ). With this, you can now subscribe to notificationClicks
In the constructor inject SwPush then use it with:
   this.swPush.notificationClicks.subscribe( arg =>
   {
     console.log(
       'Action: ' + arg.action,
       'Notification data: ' + arg.notification.data,
       'Notification data.url: ' + arg.notification.data.url,
       'Notification data.body: ' + arg.notification.body,
     );
  });
If you don't want to update you should watch the file ngsw-worker.js with Webpack or gulp or etc. And then after the line with
this.scope.addEventListener('push', (event) => this.onPush(event));
add the following:
this.scope.addEventListener('notificationclick', (event) => {
    console.log('Notification event', event);
    event.notification.close();
    var payload = event.notification.data;
    if (event.action && payload.actions && payload.actions.includes(event.action) 
       clients.openWindow && event.notification.data.url) {
      event.waitUntil(clients.openWindow(event.notification.data.url));
    }
  });