If you would like to track successful affiliate signup in your Post Affiliate Pro into some 3rd party application (like Google Analytics, etc.) then there are a few ways you can do it.
1. Use the Callback URL option under Configuration->Affiliate signup and under the General tab where you can define the URL address which will be notified about every successful signup.
2. You can do it by customizing the design of the affiliate signup page from your merchant panel where you would navigate to the Configuration > Design > Signup page theme and you'd edit the active theme and find the "footer.stpl" file. Place the following code into this file and replace the red part of the code with your custom (tracking) code:
3. The last option is to not use the default Post Affiliate Pro signup form but instead to use an external signup form which you can place to your own website and track it there.