Welcome notifications are a great onboarding experience for users who have just signed up to your site, because they give users immediate feedback on what the experience of receiving web push notifications from your site will be like.
If welcome notifications are enabled, as soon as a user subscribes to push notifications on your site, they will receive a notification thanking them for joining.
Typical Sites - Go to Web Push Settings and scroll down to Welcome Notifications.
Wordpress Sites - Go to your Plugin and scroll down to Welcome Notification Settings.
Custom Code - See Custom Code Examples: Welcome Notifications