All Firebase Push Notification parameters are available from the Google API test console. The direct link to the Send method test console – https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages/send

From the above URL other details can be navigated to.

To open the console click on the “Try it!” blue button on right.

Now clicking the blue dots in the basic JSON  will show the relevant params of that section.