Set Track Object with Camera Interactively

To set up object tracking interactively:

Select the object you want to remain stationary (that is, the one that tracks the target object).
Move the cursor over the target button () that appears next to the object's bounding box.
Drag the Target line from the button to the target object. The target object turns green. As you drag the Tracking icon from the tracking object to the target, a tracking line appears. Let up on the mouse.

Move the target object to test that the tracking is working.