Attach event to dynamic elements in javascript

This is due to the fact that your element is dynamically created, so it is attached to the DOM later, but your addEventListener call already occurred in the past.
You should use event delegation to handle the event.

document.addEventListener("click", function(e){
  const target = e.target.closest("#btnPrepend"); // Or any other selector.

  if(target){
    // Do something with `target`.
  }
});

closest ensures that the click occurred anywhere inside the target element or is the target element itself.
This is useful if, for example, instead of your <input id="btnPrepend"/> you had a <button id="btnPrepend"><i class="icon">+</i> prepend</button> and you clicked the <i class="icon">+</i>.

jQuery makes it easier:

$(document).on("click", "#btnPrepend", function(){
  // Do something with `$(this)`.
});

Here is an article about event delegation.