Outlook internally uses Extended MAPI (accessible in C++ or Delphi only).
You can use the Outlook Object Model or Redemption (I am its author), which wraps Extended MAPI.
If you need to build a task request without using the MAPI system in one form or another, you would need to build a winmail.dat attachment, which is a TNEF stream containing MAPI specific properties used by task requests.