Send a text message to an active conversation

The target conversation in the following formats:

• ConversationId: The unique ID of a conversation.
• PhoneNumber: The recipient's phone number (e.g., 14155552671).
• Channel:PhoneNumber: A combination of the channel (name or phone number) and the recipient's phone number (e.g., MyChannel:1415552671, 123456789:1415552671).

Message content.