Unfortunately ,Camtasia 9 will not record just the webcam.
You could select a very small portion of the screen.17 x 17 pixels is the smallest size selectable,See Image Below, Click to enlarge.
Record your webcam and the screen at the same time. Delete the tiny screen recording when you edit the webcam in Camtasia. The small screen recording shouldn't put a large drain on system resources and it's the only option I'm aware of.
I use a Logitech 920 webcam and the Logitech webcam recorder does a better job of recording the camera than Camtasia.