Install Windows 10 on old Macbook Pro without optical drive
Try this method. If you have any problems or questions, post a comment below. BTW, this is not the only method that works. Other variations exist, so if this one does not work, I can suggest another.

Basically, you will be creating a virtual disk to replace the DVD or flash drive that you would normally use. You will be installing Windows to boot using the BIOS boot method. Good luck!