Full Code

BackendFriend.cs

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

// Add BACKND SDK namespace
using BackEnd;

public class BackendFriend
{
    private static BackendFriend _instance = null;

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

            return _instance;
        }
    }

    private List<Tuple<string, string>> _requestFriendList = new List<Tuple<string, string>>();

    public void SendFriendRequest(string nickName)
    {
        var inDateBro = Backend.Social.GetUserInfoByNickName(nickName);

        if (inDateBro.IsSuccess() == false)
        {
            Debug.LogError("An error occurred while searching the name of the user. : " + inDateBro);
            return;
        }

        string inDate = inDateBro.GetReturnValuetoJSON()["row"]["inDate"].ToString();

        Debug.Log($"{nickName}'s inDate value is {inDate}.");

        var friendBro = Backend.Friend.RequestFriend(inDate);

        if (friendBro.IsSuccess() == false)
        {
            Debug.LogError($"{inDate} An error occurred while sending a friend request. : " + friendBro);
            return;
        }

        Debug.Log("Friend request sent successfully." + friendBro);
    }

    public void GetReceivedRequestFriend()
    {
        var bro = Backend.Friend.GetReceivedRequestList();

        if (bro.IsSuccess() == false)
        {
            Debug.Log("An error occurred while loading the received friend requests list. : " + bro);
            return;
        }

        if (bro.FlattenRows().Count <= 0)
        {
            Debug.LogError("There are no received friend requests.");
            return;
        }

        Debug.Log("Successfully loaded the received friend requests list. : " + bro);


        int index = 0;
        foreach (LitJson.JsonData friendJson in bro.FlattenRows())
        {
            string nickName = friendJson["nickname"]?.ToString();
            string inDate = friendJson["inDate"].ToString();

            _requestFriendList.Add(new Tuple<string, string>(nickName, inDate));

            Debug.Log($"{index}. {nickName} - {inDate}");
            index++;
        }
    }

    public void ApplyFriend(int index)
    {
        if (_requestFriendList.Count <= 0)
        {
            Debug.LogError("There are no friend requests.");
            return;
        }

        if (index >= _requestFriendList.Count)
        {
            Debug.LogError($"Exceeded the limit of the received friend request list. Select : {index} / List maximum : {_requestFriendList.Count}");
            return;
        }

        var bro = Backend.Friend.AcceptFriend(_requestFriendList[index].Item2);

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

        Debug.Log($"{_requestFriendList[index].Item1} has become a friend. : " + bro);
    }

    public void GetFriendList()
    {
        var bro = Backend.Friend.GetFriendList();

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

        Debug.Log("Successfully loaded the friends list. : " + bro);

        if (bro.FlattenRows().Count <= 0)
        {
            Debug.Log("There are no friends.");
            return;
        }

        int index = 0;

        string friendListString = "Friends List\n";

        foreach (LitJson.JsonData friendJson in bro.FlattenRows())
        {
            string nickName = friendJson["nickname"]?.ToString();
            string inDate = friendJson["inDate"].ToString();

            friendListString += $"{index}. {nickName} - {inDate}\n";
            index++;
        }

        Debug.Log(friendListString);
    }
}

BackendManager.cs

using UnityEngine;


// Add BACKND SDK namespace
using BackEnd;

public class BackendManager : MonoBehaviour
{
    void Start()
    {
        var bro = Backend.Initialize(); // 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)
    void Test()
    {
        // Signed up as user2 because the friend request will be sent to user1
        string user2Id = "user2";

        // Sign up as user2Id (When 409 error occurs, change CustomSignUp to CustomLogin because an ID has already been created with user2Id)
        BackendLogin.Instance.CustomSignUp(user2Id, "1234");
        BackendLogin.Instance.UpdateNickname(user2Id); // Change the nickname to be same as the ID

        string user1Nickname = "Desired name"; // Nickname of user1 (may differ depending on the user)
        BackendFriend.Instance.SendFriendRequest(user1Nickname); // Method for sending friend requests


        // Must be logged in as the user to which the friend request was sent in Step 1.  
        BackendLogin.Instance.CustomLogin("user1", "1234");

        BackendFriend.Instance.GetReceivedRequestFriend(); // Load friend request list
        BackendFriend.Instance.ApplyFriend(0); // Accept the latest friend request on the friend requests list

        BackendFriend.Instance.GetFriendList(); // Load friends list

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

Go to the next chapter