def METHOD_NAME(headers: Headers) -> str:
    """
    Build a handshake request to send to the server.
    Update request headers passed in argument.
    Args:
        headers: Handshake request headers.
    Returns:
        str: ``key`` that must be passed to :func:`check_response`.
    """
    key = generate_key()
    headers["Upgrade"] = "websocket"
    headers["Connection"] = "Upgrade"
    headers["Sec-WebSocket-Key"] = key
    headers["Sec-WebSocket-Version"] = "13"
    return key