Full Code

BackndFriend.cs

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

// Adds BACKND SDK namespace
using BACKND;

public class BackndFriend {
    private static BackndFriend _instance = null;

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

            return _instance;
        }
    }

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

    public void SendFriendRequest(string nickName) {
        var inDateBro = Backnd.Player.GetPlayerInfoByDisplayName(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($"inDate value of {nickName} is {inDate}.");

        var friendBro = Backnd.Friend.SendFriendRequest(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 GetReceivedSendFriendRequest() {
        var bro = Backnd.Friend.GetUnresolvedFriendRequestList();

        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(BACKND.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 = Backnd.Friend.AcceptFriendRequest(_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 = Backnd.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(BACKND.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);
    }
}

BackndManager.cs

using UnityEngine;
using System.Threading.Tasks;

// Adds BACKND SDK namespace
using BACKND;

public class BackndManager : MonoBehaviour {
    void Start() {
        var bro = Backnd.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)
    async void Test() {
        await Task.Run(() => {
            // 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 CustomSignIn because an ID has already been created with user2Id)
            BackndLogin.Instance.CustomSignUp(user2Id, "1234");
            BackndLogin.Instance.ChangeDisplayName(user2Id); // Change the nickname to be same as the ID

            string user1Nickname = "Desired Name"; // Nickname of user1(may differ depending on the user)
            BackndFriend.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.  
            BackndLogin.Instance.CustomSignIn("user1", "1234");
            
            BackndFriend.Instance.GetReceivedSendFriendRequest(); // Loads friend requests list
            BackndFriend.Instance.ApplyFriend(0); // Accepts the latest friend request on the friend requests list
            
            BackndFriend.Instance.GetFriendList(); // Loads friends list

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

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