Action Layout Editor for Post Action to add custom fields

I would like to add some custom fields to the post action on the case object. How can I edit the layout for the post action in the chatter component?

I've been able to do this for email by adding new custom fields to the EmailMessage object then going to Buttons, Links and Actions on the Case object and adding them to the Action Layout Editor. The outcome is shown below:

How can I do the same for post so I can have more than just a big comment box? I want to ensure certain fields values are selected before a post can be shared.