JSFiddle - React, Tailwind, and code Playground

HTML

<div id="text" contenteditable="true">
  <li>This is the first paragraph.</li>
  <li>This is the second.</li>
  <li>This is the third.</li>
</div>

<div id="savebox">
  <textarea id="textarea">Copy &amp; paste here with breaks in between each list item.</textarea>
</div>

CSS

li {
  list-style-type: none;
}
#savebox {
  float: right;
}
#textarea {
  height: 200px;
  width: 200px;
}

JavaScript

//create global variable to store copied text.
var s;
//listen to copy for your element
$("#text").on("copy", function() {
  //get selected text while coping  
  s = window.getSelection().toString();
  //customize breaklines to be double
  s = s.replace(/\n/g, "\n\n");

  //add break line to the end and more text
  s += "\n" + "- More text says you are welcome";
  //HTML will displays as plain text in textarea
  s += "\n" + "- <p>Hello world</p>";

});
// listen to paste
$("#textarea").on("paste", function() {
  //set value with your customized copied text
  $("#textarea").val(s);
  return false;
});