Full Code

BackendGuild.cs

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

// Adds BACKND SDK namespace
using BackEnd;

public class BackendGuild {
    private static BackendGuild _instance = null;

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

            return _instance;
        }
    }

    public void CreateGuild(string guildName) {
        // goodsCount entering as 10 is the number of goods types that can be used and cannot be modified later.  
        var bro = Backend.Guild.CreateGuildV3(guildName, 10);

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

        Debug.Log("Guild created. : " + bro);
    }

    public void RequestGuildJoin(string guildName) {
        var bro = Backend.Guild.GetGuildIndateByGuildNameV3(guildName);

        if(bro.IsSuccess() == false) {
            Debug.LogError($"An error occurred while searching {guildName}. : " + bro);
            return;
        }

        string guildInDate = bro.GetFlattenJSON()["guildInDate"].ToString();

        bro = Backend.Guild.ApplyGuildV3(guildInDate);

        if(bro.IsSuccess() == false) {
            Debug.LogError($"An error occurred while sending a join request to {guildName}({guildInDate}). : " + bro);
            return;
        }

        Debug.Log($"Successfully sent guild join request to {guildName}({guildInDate}). : " + bro);
    }

    public void AcceptGuildJoinRequest(int index) {
        var bro = Backend.Guild.GetApplicantsV3();

        if(bro.IsSuccess() == false) {
            Debug.LogError("An error occurred while loading the list of users who requested to join the guild. : " + bro);
            return;
        }

        Debug.Log("Successfully loaded the list of users who requested to join the guild. : " + bro);


        if(bro.FlattenRows().Count <= 0) {
            Debug.LogError("There are no users who requested to join. : " + bro);
            return;
        }

        List<Tuple<string, string>> requestUserList = new List<Tuple<string, string>>();

        foreach(LitJson.JsonData requestJson in bro.FlattenRows()) {
            requestUserList.Add(new Tuple<string, string>(requestJson["nickname"].ToString(), requestJson["inDate"].ToString()));
        }

        string userString = "Join request list\n";

        for(int i = 0; i < requestUserList.Count; i++) {
            userString += $"{index}. {requestUserList[i].Item1}({requestUserList[i].Item2})\n";
        }

        Debug.Log(userString);

        bro = Backend.Guild.ApproveApplicantV3(requestUserList[index].Item2);
        if(bro.IsSuccess() == false) {
            Debug.LogError($"An error occurred while approving the join request of {requestUserList[index].Item1}({requestUserList[index].Item2}). : " + bro);
            return;
        }

        Debug.Log($"Successfully accepted the join request of {requestUserList[index].Item1}({requestUserList[index].Item2}).: " + bro);
    }

    public void ContributeGoods() {
        var bro = Backend.Guild.ContributeGoodsV3(goodsType.goods1, 100);

        if(bro.IsSuccess() == false) {
            Debug.LogError("An error occurred during guild contribution. : " + bro);
        }

        Debug.Log("Successfully contributed goods to the guild. : " + bro);
    }
}

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(() => {
            
            // #Step 1
            BackendLogin.Instance.CustomLogin("user1", "1234"); // Logs in as user1
            // When 412 error occurs while creating a guild, the creation of the guild is unavailable because the user already belongs to a guild.  
            // Create a new user with CustomSignUp("user3", "1234") to generate a guild.  

            BackendGuild.Instance.CreateGuild("Desired_guild_name"); // Method for creating guilds
            
            // #Step 2
            BackendLogin.Instance.CustomSignUp("guildUser", "1234"); // New sign-up of user for the guild
            BackendLogin.Instance.UpdateNickname("guildUser"); // Nickname registration of user for the guild

            // The guild name entered in CreateGuild during Step 1 should be entered as the parameter value.  
            BackendGuild.Instance.RequestGuildJoin("Desired_guild_name");
            
            // #Step 3
            BackendLogin.Instance.CustomLogin("user1", "1234");
            BackendGuild.Instance.AcceptGuildJoinRequest(0); // From users who sent join requests, accepts the one who sent the most recent request.  
            
            // #Step 4
            BackendGuild.Instance.ContributeGoods(); // Guild goods contribution(addition) method

            Debug.Log("Test complete.");
        });
    }
}