You can implement the template_preprocess_field__comment()
hook in your custom module:
/**
* Implements hook_preprocess_field__comment().
*/
function MY_MODULE_preprocess_field__comment(&$variables) {
switch ($variables['field_name']) {
//field_message_custom is your custom comment field name
case 'field_message_custom':
$variables['add_new_comment'] = t('Review the book');
break;
default:
$variables['add_new_comment'] = t('Add new comment');
}
}
And now you change use the add_new_comment
variable in your field--comment.html.twig this line:
<h2 class="title comment-form__title">{{ 'Add new comment'|t }}</h2>
for this line:
<h2 class="title comment-form__title">{{ add_new_comment }}</h2>
After this clean your cache.
It seems that we will be able to change this message in the interface in Drupal 8.3. Read: You can add a review, opinion, greeting, etc. comment type but not change the "Add new comment" text