With version 4.0.0 (or below) we only allow display Customize button (Custom Your Design button) on Product Details page.
From version 4.1.0, we will support setting to display it on others page (Search Result, Compare, Featured, Manufacture, Categories,…)
To get started, go to Tshirtecommerce> Settings > Display Custom Button on.
With Featured extension, Bestseller extension, Latest extension, Special extension you need active them on Extensions > Modules list.
Result after setting is Yes: