Default Field Value based on another referenced field
You can use a hook_ENTITY_TYPE_create() to prepopulate comment fields with data from the parent entity:

  function mymodule_comment_create(\Drupal\comment\CommentInterface $comment) {
    $node = $comment-&gt;getCommentedEntity();
    if ($node &amp;&amp; $node instanceof \Drupal\node\NodeInterface) {
      if ($node-&gt;hasField('field_example')) {
        $comment-&gt;subject-&gt;value = $node-&gt;field_example-&gt;value;
      }
    }
  }