What is the correct way to copy/paste an innerHTML by javascript to a textarea input, managed by CKEditor ?

I have succeeded to implement CKEditor in an input textarea of a rails new/edit simple_form_for. It works well. But I have some javascript code which copy an innerHTML somewhere to paste it in the input textarea managed by CKEditor, and this not working. I don't find the right "id" or "class" in this input to use with a querySelector in order to paste my innerHTML. I have already tried with .content-input , #substep_content , .substep_content and .cke_editable, but it doesn't work. Someone can help me, please ? Here is the form :

<%= simple_form_for(@substep) do |f| %>
    <%= f.input :content, label_html: { class: "content-label" }, input_html: { rows: 15, class: "content-input", ckeditor: { toolbar: 'Full' } }, as: :ckeditor %>
<% end %>

When I open "page source code" :

<div class="form-group ckeditor optional substep_content">
 <label class="ckeditor optional content-label" for="substep_content">Contenu</label>
 <textarea rows="15" class="ckeditor optional content-input" name="substep[content]" id="substep_content"></textarea>
 <script>
<img width="531" alt="CKEditor" src="https://user-images.githubusercontent.com/58553969/87330638-29019a00-c539-11ea-9f35-0f3fe613c21c.png">

 //<![CDATA[
 (function() {
            if (typeof CKEDITOR != 'undefined') {
              if (CKEDITOR.instances['substep_content']) { CKEDITOR.instances['substep_content'].destroy(); }
              CKEDITOR.replace('substep_content', {"toolbar":"Full","customConfig":"/assets/ckeditor/config-dff8e9963191c33e28d6d4ccec8784c5b317df36f5ae95a4f171ca46bd9f1df2.js"});
            }
          })();
 //]]>
 </script>
</div>

Here the : Image of inspector / examine the item

galetahub / ckeditor

What is the correct way to copy/paste an innerHTML by javascript to a textarea input, managed by CKEditor ? #896