If you want to open native IOS player and play streamed video, you can use PlaySteamingVideo methods and provide steam video URL. Here is an example:
Open Youtube Video
Youtube video can be opened using OpenYoutubeVideo method. You should pass video code as a param.
You can get video code from video address in browser :
And here is an example of using:
Note: Video will be opened in the native Youtube application if there is no application on the device system which web browser will use.
An example of using can be found in NativeIOSActionsExample.cs script.