The easiest method to install Windows is to use the Boot Camp Assistant. However this method limits where Windows can be installed. This limitation can be overcome my avoiding the use of the Boot Camp Assistant, but is increases the complexity of the installation process.
The Windows installer GUI (Graphical User Interface) can also limit where Windows can be installed. Some of this limitation can be overcome by first running the
diskpart command before using the Windows installer GUI or by avoiding the use of the Windows installer GUI altogether and installing Windows by using the CLI (Command Line Interface).
An EFI booting of Windows is easier to install than one that uses the legacy BIOS boot method. Your model Mac can use either method. However, the Windows Support Software drivers you download using the Boot Camp Assistant are designed for an EFI booting Windows 10.
Your comment indicates you are having problems with the Windows installer. Can I assume you were able to create the partitions on the SSD drive using the Boot Camp Assistant and now have booted the Windows installer from the flash drive only to discover the Windows installer will not let you select the SSD drive partition created by the Boot Camp Assistant? Or, other than perhaps using the Boot Camp Assistant to download the Windows Support Software, you are installing Windows without the use of the Boot Camp Assistant application? If the later is true, then could you outline the steps you have taken so far? For example: How did you get the Windows Support Software? If you used a flash drive, then how did you transfer the installation files to this flash drive? If you avoided the use of a flash drive, then where did you put the files the normally a put on the flash drive? Once you have answer these question, I would be better able to offer the steps needed to install Windows 10.