def api_key(self, api_key):
        """
        Sets the api_key of this GlobalSignCredentials.
        Unique ID for API client (provided by GlobalSign). 

        :param api_key: The api_key of this GlobalSignCredentials.
        :type: str
        """
        if api_key is None:
            raise ValueError("Invalid value for `api_key`, must not be `None`")
        if api_key is not None and len(api_key) > 1000:
            raise ValueError("Invalid value for `api_key`, length must be less than or equal to `1000`")

        self._api_key = api_key