I'm a consultant.  I have a free ebook download.  When someone fill out my "Free Strategy Session" form, I promise them to automatically email them my ebook. 

My "Free Strategy Session" form has 4 visible fields and one hidden field to track which form they filled out. I want to add one more hidden field to track the fact that anyone who fills out that form automatically gets the ebook.  

1. I added the ebook as a product in the product module. 

2.  I'm trying to add a "product" field to the "lead" module, so that I can add it to the form as a hidden form field.  

I'm not sure how to proceed.  When I go to "Customization Modules and Fields", and select to customize "leads", I can't see any field where I could automatically update the product history of that client.  I'll have more products in the future and I'd like to keep track. 

Is there a way to do that?  What would be the steps?