Then hit "OK" and a new demo command will be added to the command list. As a helpful hint, while placing cameras, you may want to set the frame in the demo smoother first, before changing the frame in the actual demo editor. The white samples at the start and end show you the next few samples just outside your selection range.
I had no idea you could do this. You can now move that sample point by clicking "Drive Camera" and moving with the keyboard and mouse, etc.