XXX means .html and .markdown(_v2). It is supposed to mention the Chat in the picked markup. Use title or first_name + space + Lastname as the text if the input parameter name is not set (otherwise use that), and the inline mention of the chat.id.

The logic will be close to the User one, have a look at it here: https://github.com/python-telegram-bot/python-telegram-bot/blob/master/telegram/_user.py#L222

Also check out our contrib document to get started with this project, and don't forget to unit test your changes!