The program you’re looking for would be the partition manager. It is the KDE native way to make partitions with a GUI and should be installed as per standard installation.
Be aware though windows does not like to play nice when installed on the same drive as Manjaro and tends to overwrite the bootloader. So it is can be easier to have the installation on a different drive.
If you need further assistance we would need to know more about the layout of your system.