public sealed class ProjectOpenedEvent : ArcGIS.Core.Events.CompositePresentationEvent<ProjectEventArgs> 

{
  // override the Initialize and Uninitialize methods of your add-in's module to subscribe and unsubscribe to the ProjectOpenedEvent

  ProjectOpenedEvent.Subscribe(OnProjectOpened); //subscribe to Project opened event

  ProjectOpenedEvent.Unsubscribe(OnProjectOpened); //unsubscribe from the event as the module is unloaded

  void OnProjectOpened(ProjectEventArgs obj) //Project Opened event handler
  {
    MessageBox.Show($"{Project.Current} has opened"); //show your message box
  }
}