You need to have a screen recorder to do that. IMO doesn't offer screen recording within the application. Therefore, a 3rd party application is necessary to save the recorded file. To do this, I recommend installing Rec.. You can install the application through the Play Store.
Once it has been installed, configure the bit-rate, recording time, then you can initiate the video chat to start the actual recording.
Install REC. application: https://play.google.com/store/apps/details?id=com.spectrl.rec