Hi David,
Please note that the product template ID us different from Design ID.
The design ID would only be needed for an actual design to load every time, like a certain image or text or colors. If you want the product to start blank, as is common, you don’t need a design ID and you can leave that blank.
Mystyle template ID and the checkbox to enable customization are on the main Mystyle tab in the product options, near where you set all the other product info.