After a visitor completes my OptiMonk form how can I use their data in the redirect link?

If you want to use the visitor data in the popup redirect link, from the form after the visitor filled the popup form, you can use the following placeholders:

First name: [[FNAME]]
Last name: [[LNAME]]
Email address: [[EMAIL]]

You can use it in the "Redirect URL" input.
Ex.: http://optimonk.com/?firstname=[[FNAME]]&lastname=[[LNAME]]&email=[[EMAIL]]