Camtasia offers this feature if the background is a solid color. The feature is called "Remove a Color". Basically you use an eyedropper to sample the background and Camtasia does its thing.
But keep in mind that this ONLY applies to something as a layer in Camtasia. If you are wanting to produce a video where the video itself has an alpha channel, it cannot be done using Camtasia and you would need a different tool that is capable of doing this.