Using bootable flash drive with Boot Camp

I am trying to install windows 10 on my 2015 Intel Imac 5K using Boot Camp. I have a bootable flash drive with windows 10 but Boot Camp is asking for an iso file. So I created an iso file from the flash drive folder but Boot Camp says it is too large. Can I use the bootable drive directly with Boot Camp. The Internet says I can but I can't find instructions.