How to create an Opt-In Page for an affiliate offer that already has an Opt-In Page

I am trying to figure out how to create an opt-in page for an affiliate offer that already has them directed directly to an opt-in page.
The affiliate link is trying to get them to sign up for a free webinar that sells them on the product.
I want to be able to grab their contact information before they are directed to this link though.
Does anyone have any samples of such a funnel that they might be willing to share with me?
Or any ideas of what I can do?