Full Code

BackendLogin.cs

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

// Add BACKND SDK namespace
using BackEnd;

public class BackendLogin
{
    private static BackendLogin _instance = null;

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

            return _instance;
        }
    }

    public void CustomSignUp(string id, string pw)
    {
        Debug.Log("Requesting sign-up.");

        var bro = Backend.BMember.CustomSignUp(id, pw);

        if (bro.IsSuccess())
        {
            Debug.Log("Sign-up successful. : " + bro);
        }
        else
        {
            Debug.LogError("Sign-up failed. : " + bro);
        }
    }

    public void CustomLogin(string id, string pw)
    {
        Debug.Log("Requesting login.");

        var bro = Backend.BMember.CustomLogin(id, pw);

        if (bro.IsSuccess())
        {
            Debug.Log("Login successful. : " + bro);
        }
        else
        {
            Debug.LogError("Login failed. : " + bro);
        }
    }

    public void UpdateNickname(string nickname)
    {
        Debug.Log("Requesting nickname change.");

        var bro = Backend.BMember.UpdateNickname(nickname);

        if (bro.IsSuccess())
        {
            Debug.Log("Nickname change successful : " + bro);
        }
        else
        {
            Debug.LogError("Nickname change failed : " + bro);
        }
    }
}

BackendManager.cs

using System.Collections;
using System.Collections.Generic;
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()
    {
        // BackendLogin.Instance.CustomSignUp("user1","1234"); // [Addition] Sign up to BACKND (handled as comment)

        BackendLogin.Instance.CustomLogin("user1", "1234"); // [Addition] Sign up to BACKND

        BackendLogin.Instance.UpdateNickname("Desired nickname"); // [Addition] Change nickname
        Debug.Log("Test complete.");
    }
}