Hybrid webinars play an automated recorded webinar first and then real live one.
To run a hybrid webinar, you have to go to "Easy - Webinar Setup" and from "Webinar Type" > select "Hybrid Webinar".
You will be able to select the video that should play before you go live and also, a backup video in case you do not make it.
After setting up your hybrid webinar, you can add youtube live url this way
And then just wait there until your live part starts.
If you do not know how to do a live webinar, please check this article
If you want to go live first, then pre-recorded and then go live again, you can do that too. In that case though, you have to set up your event as "Youtube Live". Also, this article might help.