Please refer in selecting the Thank you template here: http://prntscr.com/j0m0hm then you can attached it in the theme settings -> Thank You Page

Then you need to put the content from Thank You Page to the Additional Script Section: https://prnt.sc/jrjivk

You can find the Additional Script Section in the backend of Shopify -> Settings -> Checkout -> Order Processing (Additional Scripts)

Once the code has been placed in the "Additional Script Section" after that then we/you need to place an order at the Store and if the Order is Placed Successfuly then we are able to see the Thank you Page with the Additional Content. :)

For more information about this page, please refer here: https://help.shoptimized.net/shoptimized-theme/thank-you-page