How can I add a download function to the "Opt-In Form, Get Free Traffic Toolkit" Template?

If not applicable to this form template. is there download functionality available to incorporate into any form?

It depends on what you are trying to allow users to download and from where.  We have a Text field that can accept HTML content (links, images, styled text, etc.). For example, you can share a file on some cloud service, then copy its link and add the link to the Text field on your form.

You can add links to the thank you page as well if you want the user to submit the form first.

If there is something else you are trying to do, please let us know a bit more detail regarding you needs and we will be happy to help.