zfs rename WD\ Red\ Pro\ 16TB/Dropbox WD\ Red\ Pro\ 16TB/Dataset/Dropbox
or as @DjP-iX said
zfs rename WD+Red+Pro+16TB/Dropbox WD+Red+Pro+16TB/Dataset/Dropbox
create the dataset first in the GUI or via CLI and escape the spaces the same way:
zfs create WD+Red+Pro+16TB/Dataset
This is also a very helpful comment, pay attention to that.
Or you rename the pool first