Good day! Is it possible to create comments using Capability card-buttons?
Yes, it is possible but it will require using the Trello REST API. We recommend using the Trello REST API Client from inside of a Power-Up as it makes it easier to walk through the authorization flow.
The REST API will allow you to update card names, descriptions, etc. Using the API requires that you ask the user to grant your application access via the authorization flow.
Sorry, I may have misunderstood the question.
You’re correct, to create a comment you need to make a POST request to the API route you mentioned.
The REST API Client is a helper method for asking the user to grant access. It is not required.