Since your SSD is new, boot from your internal Fusion drive and the external SSD connected, using Alt/Option key or switch to the internal disk using System Preferences. Format the external SSD as Free Space in Disk Utility. Connect your USB installer, power cycle your Mac, and select the USB Installer to boot from. You may see two icons from the USB, Windows and EFI Boot. Select EFI Boot. If the installer comes up, in the disk selection screen, please ensure you select the external SSD (Free Space) and see if the Installer will accept it and install Windows. Please post any error messages.
By default, the Windows installer does not have TB drivers, so it may ask for drivers.