How to make a Thank You form to track conversions?

I understand that you want to use Thank you page for tracking conversions. Unforutnately, it is not allowed to add any custom code to the Thank you page. However, you may consider creating a separate web page with your tracking code and use the option 'Redirect to external link' option in the Thank you page settings. 

https://www.jotform.com/help/211-Setting-up-the-Thank-You-Page

Hope this information helps! Please get back to us if you need any further assistance. We will be happy to help.