Your 3D Cart Secure URL enables Fomo to import your recent orders.
Sometimes this is the same as your Shop URL, but often it's different.
Log into your 3D Cart store and click Settings > General.
Copy the link in your Secure URL box. Note, this is not necesarily the same as your website URL.
While logged into Fomo, visit Integrations > 3D Cart to input the URL and enable 3D Cart.