Most browsers will only allow copying of text from a visible input box or textarea

Text area example for wordpress (jQuery)

This example overwrites the textarea value in case user has altered it

  <input id="MyElement1" type="textarea" onclick="this.select();  value="The text I want copied" >
  <a href="javascript:void(0);" onclick="CopyToClipboard('MyElement1', 'The text I want copied');" >Copy to clipboard</a>

    <script>   
      function CopyToClipboard(InputElementId, RequiredText) {
        document.getElementById(InputElementId).value = RequiredText;
        var copyText = document.getElementById(InputElementId);
        copyText.select();
        copyText.setSelectionRange(0, 99999); /* For mobile devices */
        document.execCommand("copy");
      
        alert("Copied to your clipboard");
      }
    </script>