Use the CBElement command with the Base option to define one base point for the CustomBlock reference relative to the CustomBlock geometry.
Note: A CustomBlock definition can contain only one base point element.
You cannot associate activities to a base point element. However, you can include the base point in the selection set of an activity. For example, you can include the base point in the selection set of a move activity to specify that the base point should follow the move activity.
To add a base point element:
Access
Command: CBElement
Ribbon: Edit Block > Elements > Basepoint