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

protected override bool Initialize() //Called when the Module is initialized.
{
  ProjectOpenedEvent.Subscribe(OnProjectOpened); //subscribe to Project opened event
  return base.Initialize();
}

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

protected override void Uninitialize() //unsubscribe to the project opened event
{
  ProjectOpenedEvent.Unsubscribe(OnProjectOpened); //unsubscribe
  return;
}