First, create the BeOS partition in BootIt BM. Then, during installation of:
- BeOS 4.5, you'll format and mount it for installation.
- BeOS 5.0 (CD-ROM version), you'll select it as the "onto" item. Note that the new partition will be displayed by the installation program as "Unknown File System" but will have the correct size. During installation you will initialize (format) the partition.