Unfortunately I don't know the yuilibrary but I know some Javascript.
What exactly is the problem you are working on?- do you want to know where to put the code so that it gets executed?
- do you really need the gallery method or just an on paste event listener?
- can you provide more information about the error you are facing or where you got stuck?
You could just load the following js snippet on the page where you need it ...
let itextInputField = document.querySelector("textarea");
itextInputField.onpaste = function(e){
console.log("pasted");
};