Basically you would open Camtasia and instruct it to record the screen. Then configure the recording area so it captures or surrounds the webinar. Audio is the only potential tricky part. You would definitely want to tell Camtasia to record system audio.
Be advised that sometimes system audio won't capture audio from a webinar because of the way lots of webinar software operates.
You might check to see if the webinar software itself allows recording. That would provide a backup in case the audio cannot be recorded.