The SSD (rightly) has firmware installed to identify it as an internal disk, and not as a user-removeable one (one that a USB firmware would be configured to). You don't want to mess with the firmware that the SSD has to make it look removable, because it seems from your question you dont have experience modifying it and you are likely to lose all data (Its much harder to recover from an SSD compared to a mechanical hard drive as well). This firmware is the reason why, when you plug in an external hard drive to your computer, it sometimes shows up as an internal disk and not within your removeable drives (within Windows File Explorer for example).
You can still install boot files to the second partition and register it with your bootloader, for example, using EasyBCD to choose to boot from it. You might also be able to register it in your EFI bootloader so that you can see it from BIOS and directly boot into that partition. Whatever you are hoping to achieve by booting from a USB device, you can still do without it seeing the partition as a removable drive. If you are on BIOS/MBR then I strongly recommend EasyBCD to add it to your Windows boot menu.