Altium Designer : Creating Board Outline From Mechanical Outline and Making Cutout Holes

Select Mechanical Outline. Under Properties menu, Change Mechanical outline layer to user defined board outline layer.
To create custom board shape, select board outline that has been created. On the Design Tab, in the board shape, select Define from selected Objects.
To create a cutout hole, select the desired outline. On the Tool Tab, in the Convert, select Create Boardcutout from selected primitives.