Display the onboarding welcome message after receiving a message
    that contains "start".

async def message(**payload):
    """Display the onboarding welcome message after receiving a message
    that contains "start".
    """
    data = payload["data"]
    web_client = payload["web_client"]
    channel_id = data.get("channel")
    user_id = data.get("user")
    text = data.get("text")

    if text and text.lower() == "start":
        return await start_onboarding(web_client, user_id, channel_id)