Zoom Lobby.

There is not a "lobby" for Zoom Webinars. It is a feature of the new Zoom Events platform however. 

When you start a webinar, you have practice mode to let host and panelists prep. Once you start the webinar and leave practice mode, the attendees can join. At this time, they can see and hear. If you want them to have a "lobby" experience, you can share screen, maybe play some music and announce that you will start soon if you'd like, but once they are in the webinar, they are in, you are live. 

You may also be thinking of the Waiting Room feature for Zoom Meetings. This is not  a Zoom Webinar feature. 

Hope that helps!