If you do not see the livestream button, the link has not been sent before class. Check your livestream settings to view and edit how long the link is sent to students and teachers before class.
Follow these steps to start a livestream class:
Select Schedule
Find the class you wish to stream and select the class
In the top left corner of the class roster, click the blue Start Livestream button to start streaming class
This will launch Zoom and start your livestream class