I am not sure what difficulty that you may be having.
I have recently (just this week) upgraded (doing a backup, then formatting the drive using the High Sierra Disk Utility to create a fresh drive with the AFPS formatting already in place. Installed High Sierra beta, then restored from the backup.
Ran Boot Camp Assistant, which created the extra partition that Windows 10 needed.
Here's the relevant bit for you: When my Mac rebooted after creating the windows partition, it went through a few screens to begin the Windows 10 setup.
I think about 3 screens in, I saw the window that showed ALL the partitions on the drive.
One was the partition that I wanted to use for Windows 10.
I knew which one it was because I had used an unusual size to create the partition in the Boot Camp Assistant. (50GB)
When I selected that partition, that window showed an error that it could not install Windows 10 on that partition.
(I have installed Windows 10 on four different Macs, and ALL have needed this next step!)
I clicked the "Format" button. Took a few seconds, then the window changed to show that partition as ready for installing Windows 10.
And, clicked "Next" (or whatever the button was to continue --- I think it was "Next"), and the Windows installer took over from there.
If you want to start over with Boot Camp, reboot to your macOS system. Run the Boot Camp Assistant, which will NOW give you the option to remove the boot camp partition, which you should do.
After doing that, Re-launch your Boot Camp Assistant. create your choice of size for the windows partition.
it will reboot to the Windows install setup. WATCH FOR THE PARTITION LIST SCREEN.
Select the partition that you JUST created. Click the Format button.
THAT should then let you continue with the Windows install.