Dynamic Population by query string on a page

I use a WP clean permalink.

I have a form embeded on a page and want to use the gravity dynamic population.

BUT when I try to go to the page to apply the code like this:

http://victoriachristianhomeeducators.com/become-a-member/?code=0

It doesn't work. I have an input field that is suppose to be updated so that certain members can receive a free membership.

This field the get's inserted into a product calculation field to make the price free.

Hope this helps

I did try it with couponcode and it still did not update. but on form preview it does. Which is strange.

Thanks