JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wp-content-editor-container" class="wp-editor-container"><textarea class="wp-editor-area" rows="10" tabindex="1" cols="40" name="content" id="content">&lt;a title="This is a hyperlink" href="http://domain.com/directory"&gt;This is a hyperlink&lt;/a&gt;</textarea></div>

JavaScript

var url= 'domain.com';
var append = ' ?parameter ';

$(".wp-editor-area").each(
    function(){
        var haystack = $(this).text();
        $(this).text(haystack.replace(url, url+ append));
    });