How to add a CTA button in Zoom Webinar

While you can't add a button you can pin a message (including a call to action url) to the Zoom meeting using custom closed captions and APIs. A link with any instructions would be pinned to the display of the Zoom meeting for all participants to join regardless of when they joined. If you are not a developer there is a Zoom app, Salepager, that lets you pin a call to action message to the meeting or webinar interface for all attendees to see.