That property is not exposed by the Outlook object model (for posts in any
folder).
You can set flags and so on only by using a lower level API such as CDO 1.21
(optional installation) or Extended MAPI (C++ or Delphi only) or a MAPI
wrapper such as Redemption (www.dimastr.com/redemption) and accessing the
raw MAPI property tag. In this case, for FlagStatus, it would be
PR_FLAG_STATUS (PT_LONG) with a tag value of 0x10900003.