Your Shift4Shop (formerly 3dcart) 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 Shift4Shop store and click Settings > General > Store Settings > Manage Domain & Store URL:
Copy the link in your Store URL box that starts with "https". Note, this is not necessarily the same as your website URL.
While logged into Fomo, visit Integrations > Shift4Shop and input your Secure URL.
Follow this final guide to connect Fomo from your Shift4Shop store admin.