EditorWebRequest.cs

using UnityEngine;
using UnityEditor;
using System;
using UnityEngine.Networking;

public enum HttpMethoed {
    POST, PUT, GET, DELETE
}

public class EditorWebRequest {
    UnityWebRequest request;
    AsyncOperation operation;
    Action<float> progressHandler;
    Action<string> successHandler;
    Action<string> failureHandler;

    private EditorWebRequest() { }

    public static void Start(HttpMethoed method, string url, string data, Action<float> progressHandler, Action<string> successHandler, Action<string> failureHandler) {
        var req = new EditorWebRequest {
            progressHandler = progressHandler,
            successHandler = successHandler,
            failureHandler = failureHandler
        };

        switch (method) {
            case HttpMethoed.POST:
                req.request = UnityWebRequest.Post(url, data);
                break;
            case HttpMethoed.GET:
                req.request = UnityWebRequest.Get(url);
                break;
            case HttpMethoed.PUT:
                req.request = UnityWebRequest.Put(url, data);
                break;
            case HttpMethoed.DELETE:
                req.request = UnityWebRequest.Delete(url);
                break;
        }

        EditorApplication.update += req.Update;

        req.request.SetRequestHeader("content-type", "application/json");
        req.operation = req.request.Send();
    }

    public void Update() {
        if (!operation.isDone) {
            progressHandler?.Invoke(operation.progress);
        } else {
            EditorApplication.update -= Update;
            EditorUtility.ClearProgressBar();
            if (request.isHttpError || request.isNetworkError) {
                failureHandler?.Invoke(request.error);
            } else {
                successHandler?.Invoke(request.downloadHandler.text);
            }
            request.Dispose();
        }
    }
}