Setting Redirect Page for New Sign Up Form

Setting Redirect Page for New Sign Up Form

I am creating a second sign up form; this one is to be used on landing pages on my site to which users will be taken when they click on an ad.  I want the subscriber to be taken to a special thank you page on my site after asking to join the email list.  However, I cannot find the setting to configure such a redirect.

In your support, I read:

"On the top of your page, to your right, from the customization page drop down list box you've an option to select the signup form, thank you page, signup embed button and the signup embed form and make necessary changes you need."

However, when I am in Settings > Sign Up Form Themes > [My Sign Up Form], I do not see a "customization page drop down list."

How do I set a particular url for this second sign up form?  And where is this setting located?

Thank you.