If you're using Portal Engine in version 12 you can do the followign:
- Add a new field for the item you want to track.
- Set a "hidden" css class on the form field so it doesn't actually show on the public facing site but is still part of the form.
- Set the default value to a macro. Assuming you're looking at URL parameters, you can use something like
{%parameterName%}
as the macro to capture the URL parameter value.
If you're using MVC in version 12, you'll have to do some custom javascript work to handle the click event of the form on the client side.