There is no public Apple Messages for Business API

Apple Messages for Business is not a public REST API; brands cannot obtain a direct API key. Instead, integration requires an approved Messaging Service Provider that implements Apple's server-to-server API. To go live, brands must pass an Experience Review, requiring an Apple Business Register account and live agent availability. This channel supports customer-initiated chats via Safari or Apple Maps, unlike unofficial iMessage API products that use Apple IDs or Macs to send unsolicited messages.