Postbacks can be created and edited here: Affiliate Postbacks
There are three types of postbacks that can be set.
Global - These postbacks will fire for any completions that come through to your account.
Tool Specific - These postbacks will fire only for completions that come through a specific tool.
Offer Specific - These postbacks will fire only for completions for a particular offer.
Take a look at the sample postback and the available macros on this page to determine how your postback should look.