Create a secure tunnel to port 5000 on your local machine by running ngrok http 5000.
You can visit localhost:4040 in your web browser to see requests as they happen.
In ngrok you will see a custom URL like https://<something>.ngrok.io. Update AllowedDomains, URLFormatString, and WebServiceURL in main.go based on your custom URL. Be sure to use HTTPS.
Build and run the example with go run main.go -c /path/to/certificate.p12
Visit your https://<something>.ngrok.io URL in Safari to request permission and then send a push notification, which will appear in your Notification Center.