Facebook confirmed to Social Pro Daily that it is conducting an internal test of Messenger Broadcast

Which was shared by The Next Web director of social media Matt Navarra. 

it appears that messages composed via Messenger Broadcast contain the following elements:

• A welcome message.
• The main message, which can be image and text, video and text or text-only. (The recommended image size is 1,200 pixels by 628 pixels.)
• A message title.
• A message subtitle.
• A call to action in the form of a suggested reply.

Businesses can’t send a sponsored message to threads that weren’t previously opened by their customers or prospects, and users have full control to block messages or people/businesses they no longer want to hear from.