AWS Cognito OAuth: Login request failure

问题

I want to skip AWS Cognito's Hosted UI for login/authorization. However, when I try to consume the login end point to fetch authorization-code, I'm slapped with MethodNotAllowed response. As per AWS documentation, the login end-point accepts "Get" requests only. Based on my research on this topic, I figured it was possible to use "Post" method with login credentials for the login end point (Thanks to AWS documentation).

Can someone help please me figure out the issue?

AWS Pool Settings: AWS Pool Settings

C# Code: I'm using RestSharp as the HTTP client.

        private static void CognitoOAuthSignIn(string username, string password)
        {
            var CLIENT_ID = "<client_id>";
            var RESPONSE_TYPE = "code";
            var REDIRECT_URI = "https://www.google.com";
            var SCOPE = "openid";
            var AUTH_DOMAIN = "https://<domain_name>.auth.us-east-1.amazoncognito.com";
            var USERNAME = username;
            var PASSWORD = password;

            RestClient client = null;

            // 1. Get XSRF Code
            var csrfRequestUrl = $"{AUTH_DOMAIN}/oauth2/authorize?response_type={RESPONSE_TYPE}&client_id={CLIENT_ID}&redirect_uri={REDIRECT_URI}&scope={SCOPE}";
            var csrfRequest = new RestRequest(Method.GET);
            client = new RestClient(csrfRequestUrl);
            client.CookieContainer = new CookieContainer();
            IRestResponse csrfResp = client.Execute(csrfRequest);
            var cookie = client.CookieContainer.GetCookieHeader(new Uri(AUTH_DOMAIN));
            var code = cookie.Split(';')[0].Substring(11);

            // 2. Make login request
            var loginRequestUrl = $"{AUTH_DOMAIN}/login?client_id={CLIENT_ID}&response_type={RESPONSE_TYPE}&scope={SCOPE}&redirect_uri={REDIRECT_URI}";
            client = new RestClient(loginRequestUrl);
            client.DefaultParameters[0].Value = "*/*"; // Setting "Accept" header
            client.AddDefaultHeader("Content-Type", "application/x-www-form-urlencoded");
            client.AddDefaultHeader("Accept-Encoding", "gzip,deflate");
            client.AddDefaultHeader("Accept-Language", "en-US");
            client.AddDefaultHeader("Cache-Control", "no-cache");
            client.AddDefaultHeader("Cookie", $"csrf-state=; csrf-state-legacy=; XSRF-TOKEN={code}");

            var authCodeRequest = new RestRequest(Method.POST);
            authCodeRequest.AddParameter("_csrf", code, ParameterType.GetOrPost);
            authCodeRequest.AddParameter("username", USERNAME, ParameterType.GetOrPost);
            authCodeRequest.AddParameter("password", PASSWORD, ParameterType.GetOrPost);
            authCodeRequest.RequestFormat = DataFormat.None;
            IRestResponse authCodeResp = client.Execute(authCodeRequest);
            Console.WriteLine(authCodeResp.StatusCode); //returns MethodNotAllowed
        }

来源：https://stackoverflow.com/questions/61823265/aws-cognito-oauth-login-request-failure