Код:

            
using Microsoft.Office.Tools.Ribbon;
using Microsoft.Office.Interop.Excel;
using Microsoft.Vbe.Interop;


var vbaSheetCode = Application.ActiveWorkbook.VBProject.VBComponents.Item("\"" + DateTimeFormatInfo.CurrentInfo.GetMonthName(DateTime.Today.Month + 1) + "\"");
string sCode = @"Private Sub Worksheet_Activate()
Form.Show
End Sub

Private Sub Worksheet_Deactivate()
Form.Hide
End Sub
";
vbaSheetCode.CodeModule.AddFromString(sCode);