@wsangster You don't use a specific entry point for your click handler. You write the function that will be the click handler, and you make sure your Client Script exports that function.

// ue.js
function beforeLoad(context) {
  context.form.addButton({
    ...
    functionName: "myClickHandler"
  });
}

// client.js
define([], function () {
  function pageInit() {}

  function myClickHandler() {
    alert("Hello");
  }

  return {
    pageInit: pageInit,
    myClickHandler: myClickHandler
  };
});