MainApplication.cs

using Android.App;
using Android.Content;
using Android.OS;
using Android.Provider;
using Android.Runtime;

namespace MauiApp5
{
    public static class MyHelper
    {
        public static void calendarHelper()
        {
            MainActivity.Instance.calendarHelper();
        }
    }
    [Application]
    public class MainApplication : MauiApplication
    {
        public MainApplication(IntPtr handle, JniHandleOwnership ownership)
            : base(handle, ownership)
        {
        }

        protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
    }
    public partial class MainActivity : MauiAppCompatActivity
    {
        public static MainActivity Instance { get; private set; }
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            Instance = this;
        }
        public void calendarHelper()
        {
            long eventID = 221;
            ContentResolver cr = ContentResolver;
            ContentValues values = new ContentValues();
            values.Put(CalendarContract.Reminders.InterfaceConsts.Minutes, 1);
            values.Put(CalendarContract.Reminders.InterfaceConsts.EventId, eventID);
            values.Put(CalendarContract.Reminders.InterfaceConsts.Method, (int)RemindersMethod.Alert);
            Android.Net.Uri uri = cr.Insert(CalendarContract.Reminders.ContentUri, values);
        }

        public void AddEventAt10AM(DateTime theTime, string MyMessage)
        {
            // Obtain the ContentResolver from the application context
            var contentResolver = Android.App.Application.Context.ContentResolver;

            // Calculate the start and end times for 10AM local time
            DateTime theTime10AM = theTime.Date.AddHours(10);
            long startMillis = (long)(theTime10AM.ToUniversalTime() - new DateTime(1970, 1, 1)).TotalMilliseconds;
            long endMillis = startMillis + 60 * 60 * 1000; // 1-hour duration

            // IMPORTANT: You need a valid Calendar ID. You might have to query the Calendars table to get this.
            // For simplicity, this example uses "1" but in a real scenario, you'll need to determine the correct calendar.
            int calendarId = 1;

            ContentValues eventValues = new ContentValues();
            eventValues.Put(CalendarContract.Events.Dtstart, startMillis);
            eventValues.Put(CalendarContract.Events.Dtend, endMillis);
            eventValues.Put(CalendarContract.Events.Title, MyMessage);
            eventValues.Put(CalendarContract.Events.CalendarId, calendarId);

            // Insert the event
            Android.Net.Uri eventUri = contentResolver.Insert(CalendarContract.Events.ContentUri, eventValues);

            // Optionally, add a reminder for the event (for example, alert 10 minutes before)
            if (eventUri != null)
            {
                // Extract the event ID from the URI
                long eventId = long.Parse(eventUri.LastPathSegment);

                ContentValues reminderValues = new ContentValues();
                reminderValues.Put(CalendarContract.Reminders.Minutes, 10);
                reminderValues.Put(CalendarContract.Reminders.EventId, eventId);
                reminderValues.Put(CalendarContract.Reminders.Method, (int)CalendarContract.RemindersMethod.Alert);

                Android.Net.Uri reminderUri = contentResolver.Insert(CalendarContract.Reminders.ContentUri, reminderValues);
            }
        }
    }
}