How to show subtitles during an online presentation

Hi,

I deliver trainings online using Apple Keynote on Webex. This week, there will be a delegate on my course who is hard of hearing, so I need to show subtitles throughout the presentation.

In Systems Accessibility under Accessibility, I have clicked 'Prefer closed captions and SDH' but no captions are appearing on screen when I speak.

How do I get subtitles to appear on screen when I speak and when the Keynote is in full screen mode, please?

Thanks very much.