Web Push using Firebase - Simple Push Messaging

Using Google's Firebase messaging product, we would like to send push messages to individual subscribers, not a mass push, on Chrome browsers (others to be added later). Workflow Example A user visits a target web site web site and subscribes to push notifications from the web site, a site that is owned or controlled or managed by us. Later, we would push a message to that user. We would like to use a customized Get Notification action, like a "Get Notifications" button, that would subcribe the user. When we push a message to the user, we would like the push message displayed to be either of the following two formats: 1. Message with icon, target URL from pushed message. 2. Message with icon and large image below it, target URL from pushed message. We would determine at the time we push a message which style we want to use. Our push payload will include: - Title - Body - Icon - Image (not all the time) - Click URL destination The pushed message also has the following requirements: * Requires a Click or Action to close. * The push message should be delivered regardless if the user is on the initial web site or not. Other Notes For functionality, we do not need an application to push the message. We would prefer the developer use a simple JSON post (cURL or similar) to show functionality around the requirements. Our goal is to store user tokens external to Firebase. Developer would need to create a hook for us to capture the token of the user.