[bes]

<script src="//cdn.tinymce.com/4/tinymce.min.js"></script>

https://www.tinymce.com/docs/get-sta...partofawebform

Note: if you're testing this locally, you will need to prepend https:// to urls in the script tag. For example, <script src='https://cdn.tinymce.com/4/tinymce.min.js'></script>.

а нелокально всё работает

<button class="add_text_block">add_text_block</button>
<div id="letter_body"></div>
<script src="//cdn.tinymce.com/4/tinymce.min.js"></script>
 <script>
document.querySelector('.add_text_block').addEventListener('click', function () {
var text_block ='<textarea id="first_text" name="first_text" >Easy (and free!) You should check out our premium features.</textarea>';
var doc = document.getElementById('letter_body')
var div = document.createElement('div');
    div.innerHTML = text_block;
    doc.appendChild(div);
	tinyMCE.init({
       selector: "#first_text"

    });
},false);
</script>