Lets first set up a custom property called flosp and give it the value 666:
Sub firstone()
Dim dp As DocumentProperties
Set dp = ThisWorkbook.CustomDocumentProperties
dp.Add Name:="flosp", _
LinkToContent:=False, _
Type:=msoPropertyTypeNumber, _
Value:=666
End Sub
Next we set up the header:
Sub nextone()
n = ThisWorkbook.CustomDocumentProperties("flosp").Value
ActiveSheet.PageSetup.LeftHeader = n
End Sub