How do I inject javascript in web view in ionic(android)?

I am trying to add a header bar in web view for my app. I am using the cordova inAppBrowser plugin for this. To test it I added this code:

var win = window.open( URL, "_blank", 'location=yes' );
win.addEventListener( "loadstop", function() {
    win.executeScript({ code: "alert( 'hello' );" });
});

The URL opens but I do not get any alert in the web view. What am I doing wrong here?