If you want to do that, you don't need Boot-camp assistant.
Hold down the OPTION key before the apple logo show up during boot, release it and select your Windows media.
You need to create your windows media yourself, and to download the drivers for your mac, open boot camp assistant and select the Action menu in the menu bar (Not sure if that is an accurate translation from my system), and from that manu you can download the driver package.
When you boot from windows, you should be able to partition it the way you want. Leave the space you want for macOS unpartitioned, and let Disk utility handle it.
