Using the legacy FCM HTTPS API is it possible to send messages to a device or a device group as stated in the documentation here, but their is not information regarding sending a bath of messages via an HTTP post request, we are aware that is it possible to send batch messages using the Admin SDK as shown here in FCM Admin SDK
// Create a list containing up to 500 messages.
var messages = new List<Message>()
{
new Message()
{
Notification = new Notification()
{
Title = "Price drop",
Body = "5% off all electronics",
},
Token = registrationToken,
},
new Message()
{
Notification = new Notification()
{
Title = "Price drop",
Body = "2% off all books",
},
Topic = "readers-club",
},
};
var response = await FirebaseMessaging.DefaultInstance.SendAllAsync(messages);
We are unable to confirm if it its possible to send batch messages via the HTTPS post request.