Inside each virtual room there will be a "REC" button that you must press when you want to record and press again when you no longer want to continue recording.
If for any reason you have left the virtual room and re-entered, you must remember to click the REC button again.
Please note that due to the LOPD (Organic Law on the Protection of Personal Data), students are not recorded (neither webcam nor voice) in any of the videoconference rooms.
Read: How webinar rooms work.