If you would like to track successful affiliate signup of your Post Affiliate Pro signup form into some 3rd party application (like Google Analytics, etc.), you can do it by customizing the design of the affiliate signup page.
To do that, in your merchant panel navigate to the Configuration > Design > Signup page theme.
Edit the 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: