Send an email or Slack message to a thread. Authentication is required. For Slack messages, channelId can be provided or will use the thread’s slackChannelId if available.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.
The ID of the thread to send the message to (also provided in URL path)
HTML-formatted content of the message
1CC recipients for email messages
BCC recipients for email messages
File attachments for the message
Slack channel ID. If not provided, uses the thread's slackChannelId if available (for Slack messages only)