You can turn your product to custom product through app backend. Open Customify - Product Customizer backend.
- On the main menu, click the 'Custom Products' menu
- On the custom products page, click Create New, the product search popup will be showing up
- Search the product by name and click on the product you want to add. Once you click it, the product will be added to the custom products list.
- Click OK button
The customize it button will be showing up on the product page you add to custom products list.