App dependancies:
API Key

ACF requirements:
init method/external way to set API key & scope
callback method to receive data.

WP flow:
expose scope as window.ytFetchScope
add callback method to scope.
move API key to scope [set from constant (can be null) initially]

onLoad = new Fetcher(iframe id)
var Fetcher = function(id){
var target = iframe.window
var input = iframe.sibling
target.ytFetch.callback = function(data){
input.data = data;
}
target.APIKey = 'bla';
target.data = input.val;
}