I have a custom excel content type and am attempting to create a unique file name

Sub SaveByDate()
Dim MyFileName As String
MyFileName = "PO_" & Format(Now(), "yyyymmdd-hhnnss")
ActiveWorkbook.SaveAs Filename:=MyFileName
End Sub

HOWEVER when I change the name it wants to save to MY DESKTOP and not the DOCUMENT LIBRARY - how can I grab the path? (DO I REALLY HAVE TO HARD CODE THIS???)

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
If SaveAsUI Then
SaveByDate
End If
End Sub