May here from Shopify. Welcome to our Shopify Community!
The tutorial in this discussion is meant for Venture theme, which is most likely why it did not work on your theme (Supply). Each theme has different structures in the way that it is built, so the way the code is written and placed will differ from one theme to another. That said, since Supply is a theme made and supported by Shopify, I can pass this request to our theme specialists to look over. To proceed with next steps, I will send you an email to the email address attached to your Shopify Community account.
For future reference, and in case other merchants see this post, I do want to mention that although we try our best to answer all posts as quickly as possible, the discussions on our Community page is not monitored by our staffs 24/7. If your theme is made and supported by Shopify and your request falls under our design policy, you can contact our support directly for a more immediate response.
Keep your eyes peeled for the email, Sean88! I'll send you an email very shortly.