How to enable live transcription when I join the meeting?

The meeting host must have live transcription enabled for you to ask for it.  

If they have it enabled, you should see a CC button in your meeting controls at the bottom of the screen.  From there you can request captions be turned on if they aren't already, or view them either as subtitles or in a side panel.

This page should tell you more - https://support.zoom.us/hc/en-us/articles/4403492514829#h_01EJW2XGTA3QNDGQBV81PQ07NY

Thanks

KJH