How can I autoplay youtube video in the app?

Unless something has changed in a recent version of Cordova or iOS, autoplay and inline playback is possible with HTML5 videos in iOS.

config.xml configuration options that may be useful to you:

<preference name="MediaPlaybackRequiresUserAction" value="true"/>

MediaPlaybackRequiresUserAction (boolean, defaults to false): Set to true to prevent HTML5 videos or audios from playing automatically with the autoplay attribute or via JavaScript.

<preference name="AllowInlineMediaPlayback" value="true"/>

AllowInlineMediaPlayback (boolean, defaults to false): Set to true to allow HTML5 media playback to appear inline within the screen layout, using browser-supplied controls rather than native controls. For this to work, add the webkit-playsinline attribute to any <video> elements.

From the Official Cordova Docs: The config.xml File > iOS Configuration