How to copy URL on button click?

No need to create new textarea. try to get existing textarea by giving some id ('url').

Here is the working example

function Copy() {
  var Url = document.getElementById("url");
  Url.innerHTML = window.location.href;
  console.log(Url.innerHTML)
  Url.select();
  document.execCommand("copy");
}

<div>
  <input type="button" value="Copy Url" onclick="Copy();" />
  <br /> Paste: <textarea id="url" rows="1" cols="30"></textarea>
</div>

You should not use execCommand anymore, is deprecated, use the Clipboard API:

let url = document.location.href

navigator.clipboard.writeText(url).then(function() {
    console.log('Copied!');
}, function() {
    console.log('Copy error')
});

More on it: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API