Clearing data and assigning a new value upon close

I'm creating a PO Form for our company. I have most things working perfectly: a button to save the form exports it as a pdf and then clears all data and assigns the next number and a button to clear the form does just that and assigns the next number. I have numerous cells locked and password protected so no one can change the form or fudge the numbers. However I want to have a new PO Number automatically assigned and all data cleared upon either (preferably) close or open of the PO Form with a confirmation message saying so. And I want the workbook to automatically save without giving the option.

I'm very new to macros and have built my code so far from the help of this forum and other Google searches. I am trying this "BeforeClose" under "ThisWorkbook" but the dubugger hates my lines to change the value of a cell (H3) and ClearContents of my main worksheet. I've tried "Worksheet("PO Form").Range(etc" but it didn't work wither. Help would be appreciated.

Sub CloseForm()
    Private Sub Workbook_BeforeClose(Cancel As Boolean)
    MsgBox "Closing this form will clear all data and assign a new PO number. Please confirm."

    ActiveWorkbook.Unprotect ("hidden")
    Range("H3").Value = Range("H3").Value + 1
    Range("A10:H15,E5:H5,B17:H17,A20:H20,A24:G38").ClearContents
    ActiveWorkbook.Protect ("hidden")
    ActiveWorkbook.Close savechanges:=True
End Sub

First, you "snip" appears to include the start of another sub, then the Workbook_BeforeClose macro. Make sure you don't have that error in your workbook, too!

Macros in the ThisWorkbook module are global. Anything you construct in there must include full addressing to the correct sheet. Your "Range" command has no parent sheet indicated, so the macro doesn't know where to apply that range command.

Private Sub Workbook_BeforeClose(Cancel As Boolean)

    If MsgBox("Closing this form will clear all data and assign a new PO number. Please confirm.", vbYesNo, "Confirm") = vbNo Then
       Cancel = True
       Exit Sub
    End If

    ActiveWorkbook.Unprotect ("hidden")
    With Sheets("Sheet1")     'correct this sheet name
        .Range("H3").Value = .Range("H3").Value + 1
        .Range("A10:H15,E5:H5,B17:H17,A20:H20,A24:G38").ClearContents
    End With    
    ActiveWorkbook.Protect ("hidden")
    Cancel = True
    ActiveWorkbook.Close True
End Sub