Some modern UEFI BIOSes might have the option to boot from specific partitions apart from drives, look for it on the "boot" section, there is usually a section called "boot override" that allows you to select where to boot from. Check if there are more than one partition per drive showing (apart from two entries, one for UEFI and another for MBR style), If so, you can very likely partition the unasigned space and force the PC to boot from that partition using the BIOS menu.
In general, you can't do that. Once you use a drive for boot, your computer starts on that disk's MBR, and boots whatever partition is selected. To be able to select from multiple partitions, a program to do that must be installed in that section of the drive. That program is called a bootloader. GRUB is probably the most common one (and very likely the one that the BIOS uses if its able to boot from concrete partitions), and it is widely used for dual booting systems. HOWEVER, although you can tecnically do that (partition available space, install grub on the MBR, so you are able to select between windows and SpinRite) I would say it is not advisable, It will be time consuming, risky for the data in your SSD in the likely case a minor detail goes wrong, and definitely overkill. Just buy the cheapest 8GB USB drive you can find on a local store, you will save yourself lots of headaches.