How to record system audio when recording screenshare?

When I record a meeting, I want system audio to be recorded, with the option to turn off microphone recording. I've tried looking at this page: https://support.zoom.us/hc/en-us/articles/201362643-Sharing-computer-sound-in-a-screen-share but when I click on 'screen share' I don't get the options dialogue box it talks about, zoom just starts sharing. If I click on advanced options, I get this, which doesn't help either:

I've tried looking at the preferences but can't see anything about recording system audio.

Would appreciate any help, thank you.