How can I autoplay youtube video in the app?

Why not use the Youtube Player IFrame API instead:

This way you can control events when player is loaded and changes state (playing, pause, stopped, …).

Here is the example I’ve tried that works on Android Phone:

Note: To test on real device, I’ve created a blank ionic 1.0 project with:

ionic start youtubeTest blank
ionic platform add android
ionic plugin add cordova-plugin-whitelist (this is needed to enable network calls)

pasted the code from pen and then:

ionic run android