def _set_sub_prop(self, key, value):
        """Set a value in the ``self._properties[self._job_type]`` dictionary.

        Most job properties are inside the dictionary related to the job type
        (e.g. 'copy', 'extract', 'load', 'query'). Use this method to set
        those properties::

            self._set_sub_prop('useLegacySql', False)

        This is equivalent to using the ``_helper._set_sub_prop`` function::

            _helper._set_sub_prop(
                self._properties, ['query', 'useLegacySql'], False)

        Arguments:
            key (str):
                 Key to set in the ``self._properties[self._job_type]``
                 dictionary.
            value (object): Value to set.
        """
        _helpers._set_sub_prop(self._properties, [self._job_type, key], value)