Mandatory Input based on another cell

I'm aware of validation to PREVENT entry to cells without prior conditions, but my requirements are the opposite of this.

If there is data in a specific location on a SPECIFIC worksheet, REQUIRE input in another cell on that sheet.

I've played with code to accomplish this across a workbook, which as I understand it is the only way this can work, but, I need this for only specific worksheets in a workbook. Here's the code I used via a workbook scope. Can this be applied to a worksheet scope?

Private Sub Workbook_BeforeClose(Cancel As Boolean)
    If Cells(1, 1).Value = "" Then
        MsgBox "Please fill cell A1"
        Cancel = True
    End If
End Sub