How to select a button from a interactive message with a bot?

I'm trying to get a bot to "click" a button on an interactive message in Slack (preferably as a bot, but using a user token works too).

I've found that the link to send the action information to be

My problem is I can't find any documentation for "chat.attachmentAction." Looking at the request sent when using my browser, it has one http argument: "_x_id" and the payload is a WebKitForm, containing 4 items: payload, client_id, payload_id, and token.

I'm sure if I'm just not sending the appropriate data or authentication or what. All of my POSTs return "invalid_payload" or "invalid_arg_name."

Any help is greatly appreciated.