not possible, google wants you to use store device tokens on your end and push notifications to them individually.

first edit the channel name
public static String asw_fcm_channel = "general";

then go to fcm_token() and add the line
FirebaseMessaging.getInstance().subscribeToTopic(asw_fcm_channel);

done this in postman use the following method

{
"message":{
"topic":"general",
"notification":{
"title":"MESSAGE TEST",
"body":"THIS IS A TEST"
}
}
}

The end and that's it, everyone on the general channel will receive notification