public abstract class Annotation : WorksheetElement 

using( var workbook = Workbook.Create( "AddNote.xlsx" ) ) 
  { 
    // Gets the first Worksheet; a Workbook always contains at least 1 Worksheet. 
    var worksheet = workbook.Worksheets[ 0 ]; 

    // Gets the annotations collection. 
    var annotations = worksheet.Annotations; 

    // Adds a Note in Cell B1 with the name of the person who wrote it. 
    var note = annotations.AddNote( new FormattedText( "This is the title of the document" ), "B1" ); 
    note.BackgroundColor = Color.Gray; 
    note.IsShown = true; 

    // Saves Workbook to disk; 
    workbook.Save(); 
  }