ComposeMessage.swift

private func composeMessage() {
    let conversation = activeConversation
    let session = conversation?.selectedMessage?.session ?? MSSession()

    let layout = MSMessageTemplateLayout()
    layout.image = UIImage(named: "message-background.png")
    layout.imageTitle = "iMessage Extension"
    layout.caption = "Hello world!"
    layout.subcaption = "Sent by /(conversation?.localParticipantIdentifier)"

    let message = MSMessage(session: session)
    message.layout = layout
    message.summaryText = "Sent Hello World message"
    
    conversation?.insert(message)
}