Add text message under "Add new comment" title
You can override field--comment.html.twig template.

https://api.drupal.org/api/drupal/core%21modules%21comment%21templates%21field--comment.html.twig/8.8.x

&lt;section{{ attributes }}&gt;
  {% if comments and not label_hidden %}
    {{ title_prefix }}
    &lt;h2{{ title_attributes }}&gt;{{ label }}&lt;/h2&gt;
    {{ title_suffix }}
  {% endif %}

  {{ comments }}

  {% if comment_form %}
    &lt;h2{{ content_attributes }}&gt;{{ 'Add new comment'|t }}&lt;/h2&gt;
    // you can add your markup here
    {{ comment_form }}
  {% endif %}

&lt;/section&gt;


clear the cache after add new template file