Set Content

If conditions allow, you best to use first way in follow several ways.

Use HTML

You can set innerHTML content which you want to init in div for creating editor.

<div id="div1">
    <p>inital content</p>
    <p>inital content</p>
</div>

<script type="text/javascript" src="//unpkg.com/wangeditor/dist/wangEditor.min.js"></script>
<script type="text/javascript">
    const E = window.wangEditor
    const editor = new E('#div1')
    editor.create()
</script>

Use JS

And you can use editor.txt.html(...) to set editor content after creating editor.

<div id="div1">
</div>

<script type="text/javascript" src="//unpkg.com/wangeditor/dist/wangEditor.min.js"></script>
<script type="text/javascript">
    const E = window.wangEditor
    const editor = new E('#div1')
    editor.create()
    editor.txt.html('<p>set conetnt by JS API</p>') // set content
</script>