Welcome to the push messaging codelab. The button below needs to be fixed to support subscribing to push.
Once you've subscribed your user, you'd send their subscription to your server to store in a database so that when you want to send a message you can lookup the subscription and send a message to it.
To simplify things for this code lab copy the following details into the Push Companion Site and it'll send a push message for you, using the application server keys on the site - so make sure they match.
Codelab Message Sending
Subscription to Send To
Text to Send