In the Design workspace, click Create > Create Sketch, and use sketch tools to create a 2D board outline.
Click Finish Sketch at the right end of the toolbar.
Click File > Save and save sketch to project location.
Click Create > Create PCB > Create Associative PCB and select the sketch.
In the Create Associative PCB dialog, optionally set the Origin, specify the X Axis, or Flip the X or Z axis.
Click OK to finish.
The 3D PCB is created, and the workspace changes to Electronics 3D PCB, where you can link to a 2D PCB, and add holes to the board.