How to check all cells before saving Workbook

[beginner]

There on Sheet1 are not filled three cells, B9, B20 and G13


I need a VBA macro that when closing the workbook (in event Before_Save or Before_Close) will check whether all cells filled and alert users
Is possibility of this?

[macropod]

You can do that with code in the workbook's ThisWorkbook module, like:

Private Sub Workbook_BeforeClose(Cancel As Boolean)
With Sheets("Sheet1")
  If .Range("B9").Value = "" Then Cancel = True
  If .Range("B20").Value = "" Then Cancel = True
  If .Range("G13").Value = "" Then Cancel = True
End With
If Cancel = True Then MsgBox "Please complete all required cells"
End Sub

[beginner]

@macropod thank you for answering and macro
Macro works correctly