question

Firstly, the error “HTTP/1.1 400 Bad Request” is a common API error. It cannot be used to locate a specific issue. You can print its error messages for more detailed information. The request and its callback functions could be something like this.

using PlayFab;
using PlayFab.ClientModels;
using UnityEngine;


public class LoginWithNSAccount : MonoBehaviour
{
    public void Start()
    {
        var request = new LoginWithNintendoServiceAccountRequest { IdentityToken = "[YourIdentityToken]", TitleId = "[YourTitleId]" };
        PlayFabClientAPI.LoginWithNintendoServiceAccount(request, OnLoginSuccess, OnLoginFailure);
    }


    private void OnLoginSuccess(LoginResult result)
    {
        Debug.Log("Success.");
    }


    private void OnLoginFailure(PlayFabError error)
    {
        Debug.Log(error.GenerateErrorReport());
    }
}

Besides, we are not allowed to discuss the development details of the closed game platforms publicly, such as Nintendo Switch, PlayStation, etc. If the issue is on the content of IdentityToken, you still need to submit the questions on the Nintendo private developer forum for help.