Is your system UEFI (and BIOS) or just BIOS. You have to be sure to install both systems in the same boot mode and how you partition also makes a difference.
If you only have BIOS or just want BIOS you must use MBR(msdos) partitioning not gpt. Ubuntu can use either gpt partitioning with UEFI or BIOS, but Windows only boots from gpt partitioned drives with UEFI.
Then you can just leave space or create a sda1, formatted NTFS with the boot flag for your install of Windows later. You may want to also have another NTFS partition for shared data which can be a logical partition. Windows only boots from primary partitions with BIOS and MBR(msdos) partitioning.
If you have and want UEFI Windows has additional partition requirements which you really should create in advance.
Windows Recommended UEFI-Based Disk-Partition Configurations
Both Windows & Ubuntu will use the same efi partition which should be the first partition on the drive.