Hi @Happy S,

First things first. I would not recommend doing this, but instead use annotations for it.

That being said, if you really need to do this, create a onload client script (or in the UI policy you tried already if you want it only on a certain condition):

function onLoad() {
  var notes= g_form.getControl('close_notes');
notes.placeholder = "Explain why this change is not successful?.";
}