What is the
OPAQUE-USER-ID for sending text message through api.https://developer.apple.com/documentation/businesschatapi/messages_sent/sending_text_messages
opaque user identifier
https://medium.com/swift-blondie/opaque-return-type-in-swift-f5a8887c6be6
What is an opaque ID? The opaque ID is unique to the relationship between the customer's Apple ID and the business’s business ID. A customer has a different opaque ID for every chat they have using Messages for Business. The customer decides if and when to share personal identifying information with the business. For more information, see Apple Platfrom Security
https://register.apple.com/resources/messages/messaging-documentation/faq
You can get more information from the PDF on page 159：Secure Business Chat using the Messages app