Try to use a JavaScript redirection like the following:
window.location = "vnd.youtube://the.youtube.video.url";
More comprehensively:
if( /Android/i.test(navigator.userAgent ) ) {
// If the user is using an Android device.
setTimeout(function () { window.location = "market://details?id=com.google.android.youtube"; }, 25);
window.location = "vnd.youtube://www.youtube.com/watch?v=yourVideoId";
}
If Youtube app is disabled, the timeout function will redirect you to the youtube app on the play store to let you enable the app. The second redirect will pop up and play the youtube video on Android Youtube app.
If you are already switched to the YouTube app within the timeout interval, the timeout function will not be called and you are not switched to the play store but stay in the YouTube app.