Pass true to enable unfurling of primarily text-based content.
How this field works and whether it is required depends on other fields you use in your API call. [See below](#text_usage) for more detail.
Change how messages are treated. Defaults to `none`. See [below](#formatting).
Timestamp when guest account should be disabled. Only include this timestamp if you are inviting a guest user and you want their account to expire on a certain date.