UnityEditorからプッシュ通知を特定端末に送るサンプル com.unity.editorcoroutines@1.0 を前提にしています。

PushNotificationTestWindow.cs

using System.Collections;
using System.Collections.Generic;
using System.Text;
using Unity.EditorCoroutines.Editor;
using UnityEditor;
using UnityEngine;
using UnityEngine.Networking;

namespace PushNotificationSample.EditorOnly
{
    /// <summary>
    /// UnityEditor上からFirebaseのプッシュ通知を試すことが出来るすごいやつ
    /// 
    /// curl --header "Authorization: key=<Server Key>" --header "Content-Type: application/json" https://fcm.googleapis.com/fcm/send -d '{"notification":{"title":"Hi","body":"Hello from the Cloud"},"data":{"score":"lots"},"to":"<Registration Token>"}'
    /// </summary>
    public class PushNotificationTestWindow : EditorWindow
    {
        /// <summary>
        /// FirebaseConsole→Setting→CloudMessaging→ServerKeyにある150文字くらいの長い文字列
        /// 以前の短いキーは使えない
        /// </summary>
        private const string FirebaseServerKey =
            "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";

        /// <summary>
        /// Firebase.Messaging.FirebaseMessaging.TokenReceived で取得できるトークン
        /// 端末ごと、インストールベースごとに決まる
        /// </summary>
        private const string TargetDeviceFirebaseToken =
            "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";


        private EditorCoroutine _coroutine;

        private string _title = "defaultTitle";

        private string _message = "defaultMessage";

        [MenuItem("Window/FirebasePushTestingWindow")]
        static void OpenWindow()
        {
            GetWindow<PushNotificationTestWindow>();
        }

        void OnGUI()
        {
             EditorGUILayout.LabelField("Firebase Local Testing");
            EditorGUILayout.BeginHorizontal();
            GUILayout.Label("title");
            _title = GUILayout.TextField(_title, 25);
            EditorGUILayout.EndHorizontal();
            EditorGUILayout.BeginHorizontal();
            GUILayout.Label("body");
            _message = GUILayout.TextField(_message, 64);
            EditorGUILayout.EndHorizontal();
            if (GUILayout.Button("Send PushNotification"))
            {
                Debug.Log("Pushed");
                _coroutine = EditorCoroutineUtility.StartCoroutineOwnerless(FirebasePushMessageCoroutine(_title, _message));
            }
            
            EditorGUILayout.Space();
            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            GUILayout.Label("ServerKey:");
            GUILayout.Label(FirebaseServerKey);
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.BeginHorizontal();
            GUILayout.Label("TargetDevice:");
            GUILayout.Label(TargetDeviceFirebaseToken);
            EditorGUILayout.EndHorizontal();
        }


        public IEnumerator FirebasePushMessageCoroutine(string sendingTitle, string sendingMessage)
        {
            FirebaseMessagingBody body = new FirebaseMessagingBody()
            {
                notification = new Notification()
                {
                    title = sendingTitle, body = sendingMessage
                },
                data = new NotificationData()
                {
                    some = "something"
                },
                to = TargetDeviceFirebaseToken
            };

            using (UnityWebRequest www = FirebasePushMessage(body))
            {
                var op = www.Send();

                while (!www.isDone)
                {
                    if (www.isHttpError)
                    {
                        yield break;
                    }

                    yield return null;
                }

                if (www.isNetworkError)
                {
                    Debug.Log(www.error);
                }
            }
        }

        public static UnityWebRequest FirebasePushMessage(FirebaseMessagingBody body)
        {
            //POSTだとContent-Typeが変更できない
            UnityWebRequest www =
                new UnityWebRequest("https://fcm.googleapis.com/fcm/send", UnityWebRequest.kHttpVerbPOST);
            var sendBody = JsonUtility.ToJson(body);

            Debug.Log($"willSend:{sendBody}");
            www.uploadHandler = new UploadHandlerRaw(Encoding.UTF8.GetBytes(sendBody));
            www.downloadHandler = new DownloadHandlerBuffer();

            www.SetRequestHeader("Authorization", $"key={FirebaseServerKey}");
            www.SetRequestHeader("Content-Type", "application/json");
            www.chunkedTransfer = false;
            return www;
        }
    }

    [System.Serializable]
    public class Notification
    {
        public string title;
        public string body;
    }

    [System.Serializable]
    public class NotificationData
    {
        public string some;
    }

    [System.Serializable]
    public class FirebaseMessagingBody
    {
        public Notification notification;
        public NotificationData data;
        public string to; //token
    }
}