I want the addExitHandlers function to remain in the manageUI code. You added addExitHandlers to the return statement of manageUI. That is the right thing to do. addExitHandlers is said to be a public method of manageUI.
The addExitHandlers doesn’t get moved or copied to anywhere else. Instead, the onYouTubeIframeAPIReady should now invoke (or run, or call, or execute, they all tend to mean the same thing) that addExitHandlers public method.