call exec when proxy authentication success

Hi I did google alot and still did not manage to find an answer, so I ask here, hope someone can help on this, my freeradius is running as a proxy, and I want to add a exec command when authentication is successful. so I add exec in the post-proxy { , but how to filter by the authentication result ? I want to trigger it only when Access-Accept, if dont have this filter, then how to pass the Access-Accept or Access-Reject into my script ? thanks very much