def METHOD_NAME(api_key):
    """
    Returns a TokenAuthenticator.
    The API token is concatenated with `:x` and the resulting string is base-64 encoded.
    See https://documentation.bamboohr.com/docs#authentication
    """
    return TokenAuthenticator(token=base64.b64encode(f"{api_key}:x".encode("utf-8")).decode("utf-8"), auth_method="Basic")