Generate a Service Account Key:

• In the Firebase console, go to the Service Accounts page.
• Click on the service account you just created.
• Click on the Keys tab.
• Click Add Key and choose JSON.
• Download the JSON file and save it securely. This file contains the service account key, which includes the private key and other credentials needed to authenticate with the FCM API.

Obtain the Access Token:Use below class to get Access Token.class AccessTokenFirebase{ static String firebaseMessageScope = “https://www.googleapis.com/auth/firebase.messaging”; Future<String> getAccessToken()async{ final client = await clientViaServiceAccount(ServiceAccountCredentials.fromJson( {//Your Downloaded json,} ) ,[firebaseMessageScope] ); final accessToken = client.credentials.accessToken.data; return accessToken; } }Use the Access Token in FCM API Requests:

• In your Flutter app, make HTTP requests to the FCM V1 API using the access token as an authorization header.
• For example, to send a push notification using the FCM V1 API:

String token = await accessToken.getAccessToken();
print(token);
notificationServices.getDeviceToken().then((value) async {
  var data = {
    'message': {
      'token': value.toString(),
      'notification': {'title': 'Hassan', 'body': "Body"}
    }
  };
  await http.post(
      Uri.parse(
          "https://fcm.googleapis.com/v1/projects/flutter-notifications-3c7b8/messages:send"),
      body: jsonEncode(data),
      headers: {
        'Content-Type': 'application/json; charset=UTF-8',
        'Authorization': 'Bearer $token'
      }).then((value) {
        print(value.body);
  });

});