Triggering event for GR creation

[Hendra Suryakusumah]

Hi all,
 
We're developing Workflow for GR creation.The scenario is like the
following:
 
User A create GR (tcode MB01).
Then a message will appear in User B inbox mentioned that GR# is created
when User B execute the work item then the screen display(tcode MB03) will
appear.
 
My question is how to trigger the Workflow after GR created.
 
We're in 4.6C and using obj.type : MKPF as well.
 
All information would highly appreciated.
 
Regards,
Hendra Suryakusumah