1. In your Shopify admin click Themes, and then click customize next to the theme on which your existing customizer is currently installed,
I am using the dawn theme for the demonstration:

2. Go to the existing template page that your customizer currently lives on.

- On the top bar, click on the dropdown that says Home page

- In the expanded list, either in products or pages, find the template that your customizer lives on and click on that template.

3. In the Shopify template editor left-hand sidebar, click on Apps

4. Ensure that an option similar to this is selected and then save the changes for your theme.