how to add custom fields to comments form Drupal 8?

 Manage the comment types from the Manage > Structure > Comment types

   1. Navigate to the Content types administration page (Manage > Structure > Comment types or http://thirstysix.com/admin/structure/comment).
   2. Add the "Add comment type" button.
   3. Choose the "Target entity type" as "content"
   4. Goto the content type you want to enable Click the "Manage Fields" tab.
   5. You are now taken to the Manage fields tab for your specific content type, in our example it is the Blog post. Here you will see the different field types listed. Since we have added the Comments field, it is on the list.
   6. Here you can specify several different settings.