I’m basically trying to write a C# application to send a push notification to Microsoft Authenticator app something like 2FA authentication.
The user will need to confirm via his/her phone by typing the number shown.

And I’ve got the C# code below and I can I’m able to get the access token.
I’ve created App Registration in Azure.
But I’m getting 405-Method not allowed error on Graph post part.
I think I need to give some permissions but not sure which one(s).
My understanding is I need to give Graph Device related permissions under API Permissions.

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Web.UI;

namespace YourNamespace
{
    public partial class PushNotification : Page
    {
        private readonly string _tenantId = "YOUR_TENANT_ID";
        private readonly string _clientId = "YOUR_CLIENT_ID";
        private readonly string _clientSecret = "YOUR_CLIENT_SECRET";

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                string accessToken = GetAccessToken(_tenantId, _clientId, _clientSecret);

                if (!string.IsNullOrEmpty(accessToken))
                {
                    string graphApiEndpoint = $"https://graph.microsoft.com/v1.0/applications/{_clientId}/sendCustomNotificationToDevices";

                    using (var client = new HttpClient())
                    {
                        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
                        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                        string notificationJson = @"
                        {
                            ""messageTemplateName"": ""myMessage"",
                            ""notification"": {
                                ""type"": ""basic"",
                                ""title"": ""Hello"",
                                ""body"": ""This is a test notification from your app.""
                            }
                        }";

                        var content = new StringContent(notificationJson, Encoding.UTF8, "application/json");
                        var response = client.PostAsync(graphApiEndpoint, content).Result;

                        if (response.IsSuccessStatusCode)
                        {
                            ResultLabel.Text = "Notification sent successfully!";
                        }
                        else
                        {
                            ResultLabel.Text = $"Failed to send notification: {response.ReasonPhrase}";
                        }
                    }
                }
                else
                {
                    ResultLabel.Text = "Failed to obtain access token";
                }
            }
        }

        private string GetAccessToken(string tenantId, string clientId, string clientSecret)
        {
            string tokenUrl = $"https://login.microsoftonline.com/{tenantId}/oauth2/v2.0/token";

            using (var client = new HttpClient())
            {
                var requestParams = new FormUrlEncodedContent(new[]
                {
                    new KeyValuePair<string, string>("grant_type", "client_credentials"),
                    new KeyValuePair<string, string>("client_id", clientId),
                    new KeyValuePair<string, string>("client_secret", clientSecret),
                    new KeyValuePair<string, string>("scope", "https://graph.microsoft.com/.default"),
                });

                var response = client.PostAsync(tokenUrl, requestParams).Result;

                if (response.IsSuccessStatusCode)
                {
                    var jsonResult = response.Content.ReadAsStringAsync().Result;
                    dynamic json = Newtonsoft.Json.JsonConvert.DeserializeObject(jsonResult);
                    return json.access_token;
                }

                return null;
            }
        }
    }
}