If you need to create a policy for only YouTube.com, for a YouTube Category Policy, create a combined destination object "Request URL Category" with the YouTube category of "Travel" checked, AND with the Request Application URL object using "Youtube." Don't forget to put the web application in the AND field.
Should remove Application as the statement is not for an "Application" url but just a URL, and we can add the following as examples,
Adding the following screens as example of adding the youtube URL,
As a Simple match,
Or as an advanced contains,
Also adding that this is not the only ways's to add and accomplish this,
Another options would be to have a Layer specific for youtube, (would recommend a layer guard depending on size)
and use the other category as normal in the exampled way.