Full Code

BackendCoupon.cs

using System.Collections.Generic;
using System.Text;
using UnityEngine;

// Adds BACKND SDK namespace
using BackEnd;

public class BackendCoupon {
    private static BackendCoupon _instance = null;

    public static BackendCoupon Instance {
        get {
            if(_instance == null) {
                _instance = new BackendCoupon();
            }

            return _instance;
        }
    }

    public void CouponUse(string couponNumber) {
        var bro = Backend.Coupon.UseCoupon(couponNumber);

        if(bro.IsSuccess() == false) {
            Debug.LogError("An error occurred while using the coupon. : " + bro);
            return;
        }

        Debug.Log("Coupon successfully used. : " + bro);

        if(BackendGameData.userData == null) {
            BackendGameData.Instance.GameDataGet();
        }

        if(BackendGameData.userData == null) {
            BackendGameData.Instance.GameDataInsert();
        }

        if(BackendGameData.userData == null) {
            Debug.LogError("userData does not exist.");
            return;
        }

        foreach(LitJson.JsonData item in bro.GetFlattenJSON()["itemObject"]) {
            if(item["item"].ContainsKey("itemType")) {
                int itemId = int.Parse(item["item"]["itemId"].ToString());
                string itemType = item["item"]["itemType"].ToString();
                string itemName = item["item"]["itemName"].ToString();
                int itemCount = int.Parse(item["itemCount"].ToString());

                if(BackendGameData.userData.inventory.ContainsKey(itemName)) {
                    BackendGameData.userData.inventory[itemName] += itemCount;
                } else {
                    BackendGameData.userData.inventory.Add(itemName, itemCount);
                }
            } else {
                Debug.LogError("Unsupported item.");
            }
        }

        BackendGameData.Instance.GameDataUpdate();
    }
}

BackendManager.cs

using UnityEngine;
using System.Threading.Tasks;

// Adds BACKND SDK namespace
using BackEnd;

public class BackendManager : MonoBehaviour {
    void Start() {
        var bro = Backend.Initialize(true); // Initialize BACKND

        // Response value for BACKND initialization
        if(bro.IsSuccess()) {
            Debug.Log("Initialization successful : " + bro); // If successful, 'statusCode 204 Success'
        } else {
            Debug.LogError("Initialization failed : " + bro); // If failed, a 4xx statusCode error occurs 
        }

        Test();
    }

    // A method that allows synchronous methods to be called from asynchronous methods(cannot be accessed by the Unity UI)
    async void Test() {
        await Task.Run(() => {
            BackendLogin.Instance.CustomLogin("user1", "1234");

            BackendCoupon.Instance.CouponUse("f9ffeed2c882bc1418"); // [Addition] Uses a coupon whose coupon code is couponNumber
            
            Debug.Log("Test complete.");
        });
    }
}

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