If you are looking for a payload parameter to specify that you intend the message for all your users, unfortunately, it doesn't exist.
Commonly, when sending notifications to multiple users, you can make use of the registration_ids
parameter instead of to
. However, it only has a maximum of 1000 registration tokens allowed. If you intend to use this, you can make batch requests of 1000 registration tokens each, iterating over all the registration tokens you've stored in your app server.
As already mentioned by @looptheloop88, you can make use of the Firebase Console to send a message to all the users of a specific app, but if you're planning to send it via your own App Server, the most convenient way you can do is make use of is Topic Messaging. As per the answer in the possible duplicate post I commented by @DiegoGiorgini:
Sending a message to all the phones like what you do from the Firebase Web Console is only possible from the Web Console. If you need this feature from the API you can submit a feature request: https://firebase.google.com/support/contact/bugs-features/
Another possibility is to have all the client registering to a specific topic via FirebaseMessaging.getInstance().subscribeToTopic(topicName)
In this way you can send a message to the whole topic without collecting the registration-ids manually.
However, do keep in mind that Diagnostics for messages sent to Topics are not supported.