Solution:

The Marketo integration with Webex Events does not support registration confirmation emails feature. This is by design. The attendee's information will populate on the Marketo site and the Webex site after an attendee registers for event. The event registration confirmation email will not go out even if the registration approved emails option is enabled in the 'Email Messages' section on the Webex event scheduling page.

See the workaround below:

Once an attendee registers for the event, Marketo will generate a unique URL for the event.  Once the host sends out a registration confirmation email from the MARKETO tool, the attendee will get access to the event by clicking on the unique URL in the email.